If you can do abdominal work on a physioball for over 8 minutes without stopping (like the guy at my gym yesterday!), odds are you are not using your abdominal muscles. You are probably rolling on the ball and using leg muscle power and momentum to lift and lower your torso. I find this the case with many men.... Trouble finding their abs as they offer have strong arms and legs and will recruit those and use them constantly.
Deep abdominal muscles, our stabilizers, are slow firing so it's imperative to work in a slow and sustained manner (5-20 seconds.) Try having a partner physically hold your pelvis steady or at least watch you to make sure you are not rocking your pelvis back and forth. Pace yourself for slow control. I promise you'll feel it!!
I was tempted to step in at the gym yesterday.... But figured it wasn't the best idea ;) But I'm glad to help you!