This is a massive problem of mine as well. I start my rounds and range sessions worrying about 'coming over the top' and the second half of my rounds/range sessions worrying about fat shots.
I've been scouring every resource I have available to me and I have come to the following conclusions about fat hitting (irons and even woods/driver):
1. (for right handed golfers) Weight starts on left foot and doesn't transfer properly back and forth with your swing. IE: Reverse Pivot. Practice swings with 6 iron and feet together to 'feel' weight going back and forth. The start with about 60% of your weight on rear foot with driver/woods. This promotes a better weight shift forward.
1b. (for right handed golfers) Start turning your right foot in a bit to make it easier to transfer weight forward.
2. Too close to ball. Sometimes a set up with a 'relaxed' left arm is deadly when you stand too close. On the downswing the weight of the club will straighten your left arm and you will cut into the ground as a result. Try standing a bit further away for a few swings and see what happens.
3. The body shuts down on the downswing. I fight coming over the top so i have to slow down my upper body. Sometimes I do this too much. It can also happen from fatigue. Are all of your shots fat? Or when you get tired does this become more of an issue? If your body stops rotating too much during the downswing the arms throw down behind the ball. Try Brian's 'Hitting into the Box' drill and standing upright baseball bat hits. This promotes feeling your synchronization between your arms and body rotation.
I hope some of this helps but in my opinion it covers 3 very common reasons why we hit behind or fat shots. I still haven't cured it for myself. I do believe it's tied to my fighting 'over the top' because I never start out hitting fat.
Good luck. Let us know if you figure it out.
