I've always chalked it up to being an older mom. I had Pidge when I was 35 and I just haven't really bounced back. Plus, the women in my family tend to have tummies. I'm fairly resigned to it.
I wonder if it doesn't have something to do with the muscles being moved. It seems like my ab muscles just don't work the way they used to. In my pilates class, they just seem to check-out after a few minutes. They don't fatigue, or hurt, or even release...I think. Just all of a sudden I can't feel them contracting anymore. I've been meaning to ask the instructor about it.