Check your wrist position. If it's cupped too much, or bent too far backwards, the pressure for hanging onto the ball then moves more to your thumb than fingers. Also check your timing, if your hand is getting to the line too quick, you could be thumbing it. Check your hand position on the ball, make sure your thumb is straight up and down in the thumbhole, if it's bent, your hand will slide forward in the direction of your fingers, causing you to bend at the knuckles, essentially bunching up your hand near the finger holes, which will prevent you from getting any lift. Make sure you're not bending too much at the elbow trying to crank up on it to make up for losing it off your fingers, that will make it worse. Focus on tightening your fingers, and following through, if you're short arming it, that could also be a problem. There are tons of possibilities.
That's just like, your opinion, man.