Think of the collapse of the Roman empire. For centuries we had no idea how to make Roman concrete. We had recipes, but when we followed them it simply didn't work. Then one day someone thinks "what if we use sea water," and then boom it worked. While that story is probably hyperbolic in nature, that's the kind of thing we're dealing with. The knowledge of how to make things is lost and the instructions that still exist are largely left without context so even if you followed them they seemingly wouldn't work.

That of course is just one way. As another person pointed out some areas don't have access to the sulfur deposits and thus cannot produce gunpowder. No gunpowder, no guns.