Why are my waist muscles exercising but not my stomach muscles?

Check that the position of the middle gel pad is placed over your navel (belly button).  Alternatively move the belt down slightly.

If this doesn't help, move the side gel pads to a smaller size setting and/or increase the intensity.

It is normal that the stimulation in your middle does not feel as strong as the stimulation of your side muscles.  All your muscles will be working.  As you continue to increase the intensity you will feel the stimulation in all your abdominal muscles.