Hitting the middle of the golf ball is a milder version of topping. One fails to reach the bottom of the ball for one of a few reasons: poor posture, a shallow attack angle (club ascending into impact) or simply a lack of radius (chicken winging) their downswing. To offer a prescription, I’d need a video to see which one of those a student is doing. Simply send me an iphone video with your email and I’d be happy to take a look.