[Kevin Sevcik]

This is turning into a stumper. The only advantage the rest of us have over you is that our 6" and 8" wheels have more rotational inertia than your 4" wheel. So your 4" wheel doesn't store as much energy and more of the energy for the shot has to come from your motors.

This might explain everything, though. Your wheel would be losing most of its energy at the beginning when it's just pulling the ball and starting to compress. Then it's running slower through the high compression and you don't have enough compression afterward to make up for it. If you don't have good grip on your wheels, that'd likely make things even worse, wasting what little stored energy you have in friction while the wheels are slipping.

I think you'd probably be best off by increasing the grip on your wheels and trying to increase their rotational inertia. You can do that by increasing the mass or the diameter of the shooter wheel. Or both. Or by attaching a separate flywheel to your shooter axle somewhere it won't interfere with firing the ball.