They aren't hard, exactly, but they do have to fit right. I made three, ditched one, need to refire a second, and am happy with the third. The piece with the obvious gap between the lid and the bowl? Because the cup is just a hair too long.
My friend Elisabeth introduced me to these a few years ago. She made some and I used it quite a bit. Her's were a bit larger than mine, but about the same shape as the piece on the right. For a long time I tried to make them in the same shape as my honey pots, like a bee skep, but they never worked well.
I played with the idea a little yesterday- tinkering with the shape and size, but I think once I get it down, I can crank them out. Once school starts, that is!