100% Completion

So I recently emulated Kirby & the Amazing Mirror (and Nightmare in Dreamland) and can't seem to figure out why I can't get 100% completion. :(  I mean, I already defeated the final boss, went through all the rooms, and collected every collectable. What do I need to do to finish the whole game? If it's to open every treasure chest, I'm going to have a lot of time to find them...