Hitting the middle of the ball or “thin” as it is commonly called is due to lose of posture or early extension on the downswing. This will result in the right arm straightening early and a flip with the hands through impact.
Hit shots from a chest high backswing (“L”) to low finish. This will keep you in posture through impact as well as “Sync the Swing” (arms, club and body past impact. Here are two drill that will help correct this very common mistake!! Good Luck!!