Is it meant to stop when her stuffed bar is full? I filled it up and got a wee bit of dialogue from her then can't interact with anything after that. Otherwise, I see great potential but the mini-game is a bit frustrating. I think its just a little too quick.
If you are talking about the first time her stuffed bar gets full, and you get the dialogue: "it's so tight around my stomach", you are supposed to interact with her belly with your mouse/touch if you are on mobile. Just hold the left click a little bit at the belly area, and slide it down towards her pants area.