If you sometimes hit shots fat (too far behind the ball) or thin (hitting the ball with the bottom of the club), it is most likely you are not striking the ball with the correct attack angle.
For any shot where the ball is sitting on the ground, which is every shot except for tee shots, the ball should be struck with a descending blow. The lowest point of your swing arc should be about 5-7 cm in front of the ball for most iron shots.
One way to check this is to hit a few shots from grass and see where you first strike the ground in relation to the ball. First thing you will probably notice is that there is some variation in the contact point. If you are striking the ball with a descending blow, you will be better able to hit acceptable shots even with this inconsistency.
Striking the ball with a descending blow will increase your margin for error.
When you are playing tee shots you should strike the ball with a slight ascending blow. You have a bigger margin for error because the ball is on a tee and even if your tee shot is struck with a slight descending blow it will still fly quite well.
Next time you are practicing or playing, notice where the club strikes the ground in relation to the ball.
To watch the video, click on the video below.
This is the third video in the Better Golf Series. You can catch up or review the first two videos by clicking the links below