Why doesn't it fit?

CJ received these puzzles for Xmas - he is fascinated by them and loves sitting down with me and trying to put them together.

At first he didn't really get what he was meant to do, but it wasn't long before he figured it out and was busy trying to put the pieces together. Unfortunately he doesn't always try to join the correct pieces together and can get a little frustrated.

Thanks K&P&O&F for the lovely gift!