So far I agree with everyone else - non straight leaders, lots of piecing on the top etc etc.
I have run into this problem a few times in the past - I could never figure it out. I can usually straighten it out by rerolling the take up roller - but when that does not work - I have unrolled the takeup and laid small pieces of scrap batting on the saggy side and rolled them up in the takeup. Interestingly enough - when the quilts were unrolled they were square and not wavy on the sides. This leads me to believe that it was truly in the wrap on the takeup side.
Good luck!!