Not feeling the middle gel pad is common and we have found that most users will simply need to increase the intensity. If you just started using a Slendertone belt for the first time you are probably using it at a low intensity (under 30).
The middle gel pad stimulus will require a higher intensity than is needed for the side gel pads and this is because your obliques are significantly more sensitive. The lateral muscles are used less often so that any stimulation is strongly felt.
We recommend increasing the intensity to a level that is challenging yet still comfortable to experience. As you increase the intensity you will start to feel the stimulation in your abdominal muscle. Continue to increase the intensity as you progress to maximize your workout.