I have the same problem, and I often dink and almost always catch my shots (not just tee shots) heavy, just to varying degrees. At my lesson yesterday, my instructor told me (which I kind of already knew, just reinforced it) that it was because I was lunging forward at the ball, dipping my back shoulder into the ground, thus forcing the club into the ground before making contat with the ball. This slows down your clubhead significantly, so, if anything, you want to catch your shots thinner than fatter! (I don't mind nearly as much when I lose 10 yards from a *bit* thin shot to 50 yards with a *bit* fat shot)