One of the most common causes of hitting the ball shorter than you would like is that, even though you may have a good set up and you might have a really good backswing, on the downswing you might tend to release the head of the club too early thereby using the speed you have built up before you get to the ball. This is much like a boxer that when he is jabbing has his arm fully extended by the time it gets to his competitor; therefore, it does not have much power when he hits them. If he really wanted to knock someone out he would conserve that energy all the way until he got to that person and then he would extend his arm through the body which would create a knock out punch which has a lot more power. The same is true in the golf swing. If you can maintain the angle in your wrist very deep into the downswing then release it through impact you will hit the ball much farther than if you release too early. A good drill to help you learn how to do this is the Punch Drill. What I would like to see you do is set up the ball in a normal position and make a half a backswing. When you come down you want to feel like you are going to punch the ball under a tree branch or something like that. Try to hit the ball real low which will help you maintain that angle deep into the downswing. As you can see I really maintained that angle for a long time, driving the ball down. This is the first drill that you should do to try to learn to get a later release which equals more clubhead speed and longer shots.