Possible causes: 1. Too far away from ball. 2. "Flinching", or bringing your hands and arms in to your body at impact. Possible fix: 1. Stand closer or 2. Address the ball on the heel.
Coming over the top is the cause of this problem. When the club approaches the ball from outside the ball-to-target line it will travel too much to the inside after impact. This is called an out-to-in swing path and is also at the same time very steep. The only part of the clubface that can contact the ball is the toe end. This happens all too often when a player uses his bigger muscles (especially the shoulders)and swings too hard at the ball causing an in-to-out path. To correct this problem and shallow out the plane. Stand with your feet together and make shorter swings (7 iron) with mostly the arms and make sure that you hinge your wrists in the back and through swings. This will give you a better feel for he clubhead and teach you how to swing the clubhead through the ball rather than hitting at it. You will be very surprised how far you can hit the ball with this drill and how much easier it is to hit the middle of the club. Effortless power is the secret and not powerless effort!