I'm pretty sure it get's harder which each girl you get. I got 3 no problem, and the 4th is being a right pain. One thing they don't tell you is that if you don't hit the "boobs" the explode and take a hefty chunk of points. It seems random. I had one take 5000 points away. So I've found it pain to get the combo going while hitting the boobs, especially if they appear in the middle of the net. I swear they didn't explode the first few games. Or they just added more boobs making it hard to hit all of them.