Hi Purple Planet,
Were you previously able to focus on the rise and fall of the belly, or is this a first attempt?
It's a little unusual to lose that, since it's such a gross motion (unlike the breath at the nose, which can quite commonly disappear). You could try putting your hand on the belly while sitting. Or pay attention to something else (such as where you are touching the floor).
Or, just decide to sit for a while and see what happens. If you start to notice some motion due to breathing, then focus on that. I find that approach useful if I'm getting a bit tense and controlling.