Ooh! A Down force is applied? It makes so much sense now!
The curve gets shallower on the bottom, so that is why I was getting less and less momentum.
I was also probably not very good at pressing down in a way to not cancel the existing momentum. Which eventually led to a ball resting at the bottom of the curve, not reacting to my imputs.
Maybe make the ball standing still (or moving very slow) as a lose state? So it resets and we can try again (or show a game over). Because I had to exit the game with Alt+F4.