Cool game idea!
In my opinion the ball should be "accepted" if you hit the yellow circle. Right now it's not clear how precise you have to be.
Additionally I think the game could be more fun/engaging if the player could build up some tension which then will accelerate the ball. This of course would also mean that it would decelerate over time a bit. With that you could also build some nice ball to ball interaction (think billiard/pool).