The problem is that you only have five hearts. You spend a heart every time you play a stage. Once you reach 0, you can't play the game anymore for 30 minutes, where you'll get a heart back. You can get more hearts by exchanging jewels. How do you get jewels? You buy them with actual currency.. it's so damn annoying. If you want to catch every Pokemon in this game, you'll have to spend a lot.