I was wondering if anyone else has had trouble with the use of the stability ball in the abs portion of PUB. When Cathe does the excercise where her shins are on the ball and her arms are holding her body up, then she rolls the ball forward and backward, the ball just rolls out from under my legs. Does anyone have any tips? I'm wondering if it's possible that I have too much air in my stability ball or maybe I just need to practice.