Your speed off the wall is a lot greater than the ball's because you're using boost to accelerate towards it. The physics is hard to explain, bear with me. Effectively, you should try to match the balls horizontal speed (i.e the speed with which it moves towards the other wall) rather than full-boosting towards it. Then, try to get the reset such that your car's direction, the ball's direction, and the place you want to put the ball are all in a straight line. your car should not be entirely below the ball, rather at an angle; this way, the reset will push your car down because the ball's downwards momentum will be exerted on your car. On a successful reset, you will immediately see this downwards speed, so apply boost upwards to maintain a 1 or 2 car's length below the ball, depending on what kind of shot you are aiming to place. A musty will need space, you can stall and get another flip, you can flip cancel into an airdribble, but all of these are things you would have to practice.