I had similar issues finding matching items and not receiving the check mark even when I did. It seems to happen when there are duplicate items required in the cauldron. It's a really nice-looking game and the mechanics work well for the most part. I would like to be able to trade my items with the chest without having to throw them out, since I'm always scrambling to cycle items as quickly as possible. More inventory slots would help this too, I suppose.
Edit: my hypothesis about the duplicate items was incorrect, it just happens randomly. I want to see the ending but I keep either not being able to find all the items or it doesn't accept them!