Although there can be lots of reasons for hitting behind the ball or ‘fat shots’, leaning back and casting (early loss of angle between the shaft and the left arm) on the downswing tend to be the most common – causing the swing arc to take place too early resulting in the club striking the ground first.
The reaction too this for many players is to position the ball further back thinking this will help the problem. Unfortunately all this allows is for the real problem to continue and become more ingrained.
It might be surprising but if you struggle by hitting the ball fat a really good drill is to place the ball further forward.
•Take your normal set up with a seven iron
•Line the clubhead and ball up with your left breast (or right if left handed)
•Roll the ball forward towards the outside of your lead foot
•Bring your club back to the original position
•By doing this there will be a requirement to move forward in order to hit the ball, in turn moving the bottom of the swing arc
With practice and repetition you should start to feel the correct movements, which you should then try to emulate when the ball is moved back to the correct position resulting in fewer fat shots.