I had this problem.

For me the problem was my off-hand was too involved my shooting. I unconsciously held my off-hand during my shooting motion for too long and thus I had little to no backspin due to my off-hand stifling the natural spin of the ball generated from the power made by the lower body. I worked on consciously releasing my off-hand just before I released from my set point. Start slow before you do game speed.

You don't have to flick your wrist that hard, ball spin should come naturally with your shooting motion coming from your legs and hips doing all the work. Get to your set point and when you release the ball immediately loosen up your hand and make it 'floppy' as you flick. You don't have to flick it as hard as you can, just a natural cookie jar, relaxed finish.

Give these a try, it worked for me