This is a behavior I myself have, and it often boils down to not wanting to use consumable items until my class features -- daily, short rest, or whatever -- have been burned. I hate to use something non-recoverable before something that is recoverable, and even then I feel like it's a waste of the item if the battle ends right after I use it and it didn't have a significant impact. (That is, even if my spell scroll contributed to the battle, if it wasn't an earth-shaking change to the fight, I'm left feeling like maybe I should've just used a cantrip instead.)

In addition, there's often a sense that the action cost is too high, for some items. A potion of healing that recovers less than the damage I took in a single round of enemy attacks doesn't feel like I'm making a good choice. I'm likely to try to push through even with 2 HP left, because spending an action to get myself up to 9 HP feels silly if the enemy is dealing 8 to 12 damage per hit. With or without the potion, I'm just as unconscious either way, and attacking instead might just end things here and now. And after the fight, do I really want to heal with a potion? I'm pretty beat up and we're out of healing spells, so we could find a way to sleep for the night instead...

This tends to get worse over time, as well. The longer you hold an item, the less useful it is, as the opportunity cost of using that item becomes higher. As the player character levels up, the consumable items you got several levels ago become weak compared to your improved innate powers. You're even more likely to say, "Why should I waste an action on this?!"

So how can you deal with this? I can think of a few potential options.

Give out fewer consumable items, but choose stronger ones.

If the issue is feeling that consumables aren't worth spending unless they have a big impact, play into that. Instead of giving the players four potions of healing, give them one potion of greater healing that really can take somebody from near-dead to full HP (adjust which potions I'm talking about to fit your party's level, of course). Instead of handing out a selection of below-level spell scrolls, let them find a scroll for a single spell that's a level or two higher than the players, something they need an ability check to cast. It's a risk, but if it works you turn the tide in a serious way.

If the players' hesitation is indeed based on the "save it for a rainy day" effect, then this could have the unintended effect of making the players even more hesitant to use their sweet item; but on the other hand, the fact that the effect isn't that great can be a big part of it never being "the right time" to use the item.

Put a time limit on the items.

I really like the idea of, say, healing potions that are only good for a few days before they go bad. As you said, it wouldn't make a lot of sense as treasure in a dungeon, but as something a local apothecary whipped up in thanks for the party's actions, it would be a great fit.

It wouldn't be a permanent thing, but you could place an adventure in an area with a pervasive aura of rot or a drained-magic area where non-permanent items start to degrade the longer you stay, so the items have to be used or lost. This could help get the party into the mind-set of using up their stored items, at least.

Remind the players.

Sometimes it's not that you really are thinking, "Oh yes, my feather token could help here -- nah, better not." Sometimes it's just that you forget the item list on your character sheet when you're in the middle of combat. You're deep in "What would a Cleric do here?" mode and don't really consider that second sheet that's off to the side of your spell list. If called on it later, you might come up with an excuse like, "Well, it wasn't the right time" rather than admit that you just forgot you had the dang thing. So sometimes maybe it'd be helpful as DM to just say, "Hey, remember you have an item that would be perfect here."

Provide NPC companions who can use the items.

If the problem is less "saving for a rainy day" and more "action economy and underwhelming effect", an NPC companion who doesn't have other options at hand might be a good way to burn off those items. An apprentice spellcaster who only has level 1 spells, but can use a scroll, wave a token, or feed potions to unconscious party members can be a good way to get use out of those items without forcing the player characters to actively use them.

Let them sell the items.

In some cases there's just nothing you can do. Like your stack of 48 hi-potions in Final Fantasy, sometimes you just have items you will never get around to using, and that's just the way it is. It's not my favorite thing, but at some point, maybe you just need to give the players a way to convert their loads of consumables into something less consumable that might actually make them excited.

Allowing them a way to convert a whole bag full of stuff they don't care for into a single wand of web or something may not be a bad idea; charged items often fix a lot of the problems I discussed, because charges feel a lot more like "use it or lose it" and are now more renewable than, or at least as renewable as, daily character resources. If you do this, I suggest limiting their options, and picking out items to offer that have more than merely combat use, which can help break out of that 'action economy' problem.

Another option is gifting those items to NPCs. Instead of selling them for gold or in exchange for other items at the Fantasy Swap Shop, handing over "weak" items to a local noble or organization might earn them boons and good-will that money might not be able to match. A bag of trinkets that the party doesn't really want could be a huge benefit to an army or constabulary, and might earn them favors, letters of introduction, or anything a bribe could do.

My players, and I, have this behavior as well. When I analyze why we do this, it's because consumables are non-recoverable, while spell slots and features are recoverable on rests. You'll want to use recoverable resources first before start whittling down the non-recoverable ones. When you run out of recoverable resource, you start using non-recoverable resource. How to accomplish this?

1. Increase the rate of resource consumption. This can be done by increasing the number of encounters and the difficulties. Remember that encounters are not necessarily combat, but there are social encounter and puzzle and trap encounters.




2. Target resource which they have none or only a few of. If a door can only be opened using a strong enough fire spell, you'll definitely want to use that sweet fireball scroll you've just looted from the enemy wizard. It might also can only be opened by pouring potion of healing? Or it takes a lot of blood to fill the pool to find the renowned Blood Sucker Dagger, and you need healing to recover?




3. Control resource recovery. The simplest method is by not letting them having long rest benefit easily. In a dungeon, you might want to only allow short rests and not long rest because of the danger. Even short rest can only be done sparingly. In this question, there are methods how to control how they rest, for example by changing rests to 8 hours, and can only be done in an inn/city.

If players perceive scarcity, they are more likely to hold on to items. If they do not encounter many shops or find much gold, items will hold more apparent value. This is basic economics and lots of players will fall into this pattern of miserliness.

Make sure to illustrate the availability/abundance of goods as players move through the world, to mitigate their fear of scarcity. When they are in a market, the vendors can shout "Get your fancy adventuring items here! Best in town!" Have the PCs find plenty of gold when they are questing, in hoards, on bodies, as rewards for jobs.

You can also demonstrate NPCs--either combatants or non-combatants--making use of items to drive home that items are designed to be used.

There are many ways to encourage this behavior

Make consumables abundant

One method you can use is to make make consumables abundant. If they can always get some more healing potions at their local magic shop, and already have 5 from the treasure chest they looted, then they might be willing to use one when another character is making death saving throws. Its a lot easier to use something if you know you can easily get another.

Create a good opportunity for them to use it

You mentioned that they wait for the perfect opportunity to use such items. In that case, creating that opportunity would make such uses more frequent. If a character needs to make a diversion, a burning hands spell scroll would be useful. So put them in a position where a diversion needs to be made.

Make them not super amazing

It is pretty tough so use a once-in-a-lifetime-god-tier magic item, because it is worth a lot of money, and once it is gone, it is gone. It is a lot easier to lose less expensive stuff.

Potential buffs to items?

I know that this is the opposite of what I just said, but this is helpful in some scenarios, as a weak item such as a chromatic orb spell scroll is not good enough to use in combat. A common house-rule is that potions can be used as a bonus action instead of an action, which helps make them much more practical to use in combat.

Regarding potion consumables, I have had good success by implementing a house rule that allows a PC to self-administer a potion as a bonus action. By making this simple change, a player can more easily make the decision to use healing/buffs from potions when they don't have to sacrifice their entire turn to do so.

There are a few calculated risks involved I'll mention at the end, but you might try:

Make them an offer they can't refuse

By which I mean that you may need to create situations which reward creative thinking; situations which may look like they have no escape from. Situations which you may have created thinking "I have no idea how or even if they can deal with this; let's see what they come up with." You want these to be situations where their usual allotment of recoverable resources, like spell slots, are almost surely inadequate to resolve the situation and secure their safety; there's probably no time for them to recover them, anyway. This forces them to look to consumables to save the day.

Some examples:

• The party has successfully stopped Evil Organization X from summoning Horrible Abomination Y! Mostly. Horrible Abomination Y is still partially manifested and has several turns worth of actions they can take before they recede into the abyss they came from. Very deadly turns, because this thing is way beyond anything they can handle. Plus there are still plenty of members between them and the exit who are unhappy, and possibly a little stark-raving insane and stricken with bloodlust (either they were already that way, or the presence of the abomination has done less than beneficial things to them). The party needs to escape now, the faster the better, lest they all die. But access to other planes has been sealed here, perhaps due to the summoning, so they can't use things like teleport that rely on other planes. A solution may include:
  
  

  
  

  The party has a Feather Token: Oak tree. And druids just happen to have a spell which is basically "teleport from one tree to another", which is not actually a teleportation effect and does not require movement through other planes. If the party has a druid with 6th level spell slots, they may already have this ready to go, otherwise a party druid or high level rogue (depending on how you as the DM adjudicate the matter) may just whip out a spell scroll for it. In this latter case, bam, you've got them to use up two consumables at once!

  
  

  
  


• Horrible Monster Z, who is way beyond what they can handle and shrugs off their spells and attacks, is rampaging through a city. The party needs to try to evacuate citizens (and themselves), and maybe contact someone that can help. What can they do?
  
  

  
  

  Injured citizens can be healed with the party's copious supply of healing potions. A quick thinking caster can have people jump into a portable hole, then use a wand of dimension door to zip hundreds of feet away. The longer you decide the people inside can survive without problems, the more casts in succession they can use. Try combining Water Breathing and a decanter of endless water to make that easy! A staff of the magi can give them extra casts of plane shift to evacuate people, and/or passwall to quickly create escape routes. A druid and a tree can evacuate a bunch of people at once with Transport via Plants (which may be on a scroll). So on and so forth. And wands/scrolls can be used to contact powerful NPCs, if there are any. And have you ever wondered what happens if the monster steps in a puddle of sovereign glue?

  
  

  
  

Caution

Some players just need a little incentive to unleash their inner creative munchkins. There are certain magical items that can be used to hilarious and impressive effect in the right hands. But not all players will be good at solving the unsolvable, and not all parties will have someone who figures something out, or will particularly enjoy this sort of thing. Be prepared to give them some time to brainstorm a solution. You may or may not need to be a little lenient on rules to let the Rule of Cool/Fun take priority. Every group I've been with has enjoyed this sort of situation immensely, with it bringing them a great deal of satisfaction to bypass the DM's carefully laid plans, or to thumb their noses at an Eldritch Abomination as they run away with its loot. As the DM you will have to assess whether this kind of situation is appropriate for your group.

Steal them.

Magic items, especially consumable ones, are highly concentrated forms of wealth. Plus, they are often carried to be in easy reach so that they can be used at the “right time” with minimal fuss. This makes the prime targets for a pick-pocket, thief, or bandit. The more players hoard this sort of item, the more they become an appetizing target. This increased risk of losing the item without receiving any benefit, or of even having them turned against them, should make players more willing to use them while they can.

Example: Bandits raid the party’s campsite at night. They aren’t interested in a straight up fight, but a hit and run. Perhaps they cut the tethers on the party’s mounts and spook them. Then , while the party is trying to recover/calm the loose horses, one or two sneak in, grab something dangling from a pack and then hightail it out of there.

Of course, be fair about this. Especially at higher levels the players should spot the urchin pick-pocket without much trouble and be able to stop them, but even foiling such attempts places a cost on the players that should encourage them to use the items more often.

Posting this to "think out loud" and generally contribute to the community conversation. The answers so far have done a great job at illuminating the topic, but I want to try to tie things together.

To be effectively used, I feel an item must:

1. Have a specific role the characters can't replicate.

Giving a Wizard who only knows Magic Missile a Scroll of Magic Missile is fitting to practically one situation of use: the Wizard has run out of spell slots and wants to cast another offensive spell.

Giving a Wizard who only knows Magic Missile a Scroll of Ray of Sickness fits two situations: the Wizard has run out of spell slots and wants to cast another offensive spell, or the Wizard wants to try a different strategy and focus on debuffing an opponent as well as damage.

An effective item should seek to increase the player's repertoire of abilities, and thus strategies. It is a short-term way to effectively play as a different character. A healing potion can turn a Barbarian into a Cleric for a turn, instead of their usual ripping and tearing. A Scroll of Water Breathing can turn an offensive Sorcerer into a utility Wizard for a turn, instead of their usual role as the glass cannon.

2. Create no feeling of regret in the player when used.

The key thing keeping items unused, in my experience, is the players' anticipation of a metaphorical bull market, or a situation in which the item will be perfectly utilized. The players are cognizant that, while they are in possession of this item, they can execute a certain ability they otherwise couldn't. Therefore, they want to make it as useful an event as possible, such that they never think to themselves, "I should have waited to use that item here."

The problem is: that situation may never come, and the item will go unused. So this can be resolved in a couple of ways:

• Give the players that situation of perfect use.

Potion of Giant's Strength burning a hole in your Bag of Holding? Well, the temple is coming down all around you, and your party needs to do a lot of strength checks to move the rubble in order to get out in time.

Scroll of Water Breathing that just never seems to be what the kids are into these days? Well that castle you are infiltrating is heavily guarded, with the exception of the underground waterway that, well what do you know?! Your party can now traverse like fish.

Very important caveat here. Do not create this situation with the only solution being using the item. Giving the situation another solution ensures the player's not getting the hint or still (frustratingly) being shrewd with the item won't be the end of them. This time.

• Give out items plentifully (though not excessively so).

If players start to recognize that many loot drops or treasure hoards will have one-use items, or that shops tend to have these items in stock, they know that using their items is not a permanent departure from temporary abilities. Just avoid going overboard or you risk either overpowering your players or overwhelming them with choices ("I became a fighter because I didn't want to think about 15 different spells.")

• Give certain items an expiration date.

There is no remorse in using an item if one minute from now it will be useless. Let the player who blew that Nature check out of the water recognize that flower grants Jump to the player, but only for the next day.

Disclaimer: I used the tactics below on Pathfinder and D&D 3.5, but they work on D&D 5e as well with some tweaks.

I solved this issue by doing two things.

Firstly, I reduced by a fifth the cost of all consumables - scrolls, potions, wands, etc.

Then, I added in those house-rules:

Empowered/Mixed Potions: Fiddling a bit with alchemy, a player could combine a few different potions on a single vial, making it so that they could gain all of their effects when using the new, improved brew. This solves the Action Economy issue - Bob the Bard isn't likely to use an action to chug down a Health Potion to recover a few hit points, but he is more likely to use that action to drink a Fortified Health Potion, which is actually five regular Health Potions rolled into one. Combining Potions together is something that anyone trained in Alchemy could do, but it needed a check and for every potion threw in into the mix the CD went up.

Quiver of Wands: I gave my group a homebrew item that was, essentialy, a quiver that enabled the wielder of a bow to add, into his arrows, the effect of one of the wands stashed in such quiver, thus spending a charge. That made finding things like a Wand of Shocking Grasp actually exciting, because each new wand added more tools to the repertoire of the ranger of the group.

Tandem Spellcasting: My spellcasters got a new class ability at lvl 6 that let cast a spell from a scroll at the same time they were casting a spell from their slots, once a rest per every 6 levels. This had a catch - the two spells had to cast at the same target, and the caster needed a concentration check for them to not fizzle. Effectively, this boosted their damage or healing or "buffing" without costing another action, which made usually not-so-nice scrolls very attractive to them.

Those things allowed more options to the game, and gave them more resources to make up creative strategies. Using Tandem Spellcasting, for example, a Mage was able to deliver Resistance to Fire to a partymember alongside a Fireball - the first spell helped him to resist the damage while the second blew up the enemy mooks around him. Mixed Potions allowed our mage to create very interesting "buff kits", like the Red Dragon Emergency Kit, which threw in Fire Resistance, Fear Resistance and Dexterity bonus on a single kit.

The Quiver of Wands was the tool that enable our ranger - an otherwise very meh character - to become a very powerful and versatile scout, able to send an arrow with Magic Mouth to deliver an warning, blast foes with arrows charged with Shocking Grasp or Flaming Hands, or even guarantee a hit with a Magic Missile-Empowered arrow.

Yes... this is probably unbalanced. That said, those changes to the consumable mechanics gave a bunch of new options to my group, and thus I could throw more interesting stuff against them to see what would happen. More importantly, however, is the fact that opening a chest and finding a bunch of vials started to feel exciting once again.