Question about eating fiber and poop?

I absolutely love Raisin Bran and Frosted Mini Wheats Cereal, and I could pretty much live on them. However, when i eat too much of them and not so much meat or cheese and that sort of thing, i tend to get constipated. Why would eating a lot of cereal constipate me? Fiber is supposted to help keep things moving, not get them stuck!