If they used the consumables, I would provide them with more. How can I make my players realize this fact and start using their scrolls and potions more freely?
Give them more anyway
It seems that people may have missed a premise in your question. Your players don't sound like they're hoarding, it sounds as though you've given them a small number of items and they haven't used them.
People value scarce resources (at least, if they have the ability to delay gratification) and players are often trained into the thinking that "I've been given a thing, if I save this thing the hard part of the game will be easier."
Your players will always (based on the behavior you've already seen) keep a "spare tank of gas", so the idea that you'll convince them to do what their personalities dictate they will not do will be an unsuccessful endeavor.
Once your players have "enough" extra gas, they will probably use it. But, for the following point, don't be disappointed if they still hoard them.
The main issue I have with this is that I give them consumables in order to make them be able to take on tougher foes, and because they avoid using them all the time, most of the slightly tough encounters I prepared become much harder to get past.
Give them more varied single-use items
First, your players are deliberately increasing the game difficulty, but are overcoming challenges. If they aren't losing characters then your job is just easier as a DM and they're (presumably) enjoying themselves.
Don't fix what ain't broke!
But consider this; what if you gave them items that feel truly important that they need to save? Putting these next to the items that are less powerful makes those items seem more consumable as they are a tier lower.
An item I recently made up for my party was "Orb of Replenishment" - a consumable that refreshes all X-Per-Day abilities (but not spells) for the person who breaks it. Brimming with ideas, they see how powerful that could be and will probably hoard it for the big fight they see coming (or between fights.) This obviously makes some of the random things like Potion of Shield of Faith a lot less important simply because it sits next to something so powerful.
Track weight or use an "item belt" style limit
A lot of people get weird about tracking inventory/weight/etc. but if you're willing to hold your players to it then excess items get left to the wayside. I'll say this - I'm way more likely to hoard items in a system where I have no limit (because I can probably get through the game without using them; but it makes the harder part easier.)
When a system has weight penalties (say oblivion) and you have potions that weigh 1 lbs; if they aren't that effective you'll definitely use them at your earliest convenience. The reason? It's nice-to-have and it helps you, but it actually feels like a relief to stop carrying it.
An different example of this can be seen in Diablo 2. You can carry many potions and a diverse array of them, but once you fill your belt you're quite willing to use a potion because you know you'll find more. If you make your potions prevalent (as the first part of my answer and as others have mentioned) you can maybe get them to use them; but if you also limit the number they can hold all the sudden they can be "full", in which case using potions so they have room to carry more becomes attractive; because otherwise they just waste resources by dropping them by the wayside.
An issue in your current campaign (..probably)
Your players are running into encounters that are difficult but doable when they don't use their potions. They may be considering these 'somewhat difficult' encounters as 'normal' and then be saving resources for the BBEG because if these battles are hard enough to be difficult, we're going to need those.
Thus I present you with one more premise:
Give them more varied difficulty
As presented above, the psychology may be snow-balling itself because of the fact that the encounters are difficult but doable. Give them some easier encounters, then give them one harder than the ones they've done so far (just a tiny bit.) That may be a cue that "this is what we saved our resources for!"
This could take a few sessions to get them doing.