I always viewed it that the Patron was teaching secrets in exchange for...whatever it is the Patron wants in exchange. Their soul, their servitude, a slice of pie, whatever. In fact, what I usually do when DMing is explain that the Patron will trade power (a level in Warlock) for a favor, usually asked for up front. In order to gain that level, they have to do something for the Patron. In fact, I'd probably start making it so they are told "Here is your new power (level up to level 5! Yay!). If you want another, I require....a tunafish sandwich! With extra mayo!"
So, before they can level up again, sometime in the next few sessions, they have to find a way to get a tunafish sandwich with extra mayo, or they can't level up as a Warlock. But oh no, the only producer of Mayo has just gone out of business! What is he going to do!
You get the idea. If they can't, or won't, do whatever it is the Patron asked, they can't level up as a Warlock. They still get whatever they learned. They just have to pick a different class, now.
So, before they can level up again, sometime in the next few sessions, they have to find a way to get a tunafish sandwich with extra mayo, or they can't level up as a Warlock. But oh no, the only producer of Mayo has just gone out of business! What is he going to do!
You get the idea. If they can't, or won't, do whatever it is the Patron asked, they can't level up as a Warlock. They still get whatever they learned. They just have to pick a different class, now.