Originally Posted by Pogo the Clown
It could be as simple as overtraining them.
The Abs are muscles just like any other part of the body and need time to recuperate from a heavy-intense training session. Training them every day doing numerous sets and reps, plus the workout they get when you do all of the other power excercises in your routine is overkill.
If you keep pounding them day after day with any considerable rest you wont see much if any improvements as the muscles don't have time adapt and grow. If you following an intense training program you should be training your abs anymore than 1-2 a week tops. In this case less is more.
I hear that rest is more important for arms, chest and back, but you can do abs and legs every day.
Someone in my gym was telling me that we evolved to be able to be able to move continuosly all day, so our legs and core are built for stamina. But in the past we used our upper bodies for short, intensive efforts, like throwing spears or carrying things short distances. So according to this thinking, you can train legs and abs every day, but with low weights and high reps, but you train the rest with breaks, and with heigher weights.