first things first. as always, make sure your set up and posture are correct. ball too far back in your stance? standing too tall or hunched over?

next, check the transition from backswing to downswing. chances are your head is moving forward as you start your downswing, causing your weight to move forward eratically and the club to dig down behind the ball instead of sweeping it off the turf.

try this: make a very slow motion backswing (always remember when doing this drill to be aiming at something as well, whether it be a dandelion or a patch of carpet) when you get to the top, feel that your left shoulder is separating from your chin to start the downswing. this in turn will cause your right elbow to come down to your body and your weight to gradually shift forward. this will get your club on the correct plane on the downswing. when you practice this drill, just work on that transition very slowly by stopping, bringing it back up to the top, transition again, bring it back up, then transition again and keeping slow motion in mind, swing through to a full finish. (doing a slow motion swing trains your muscle memory just as well as a full speed swing, and is probably better because you're practicing the correct technique)

by the way, use this technique with any club, and youll start hitting much purer and straighter shots