Do you suffer from a lack of power specifically with your irons? It could be because your arms become disconnected from the chest during the swing. When the arms move independently of the body, it can be difficult to use the larger muscles, like the shoulders and legs, to generate the power, as TG Elite Teaching Pro Gareth Johnston explains in this golf video tip.
Disconnection can also change the plane of the swing, leading to a more glancing blow on the ball and poor distance and direction. This simple drill should go a long way to fixing this problem for good.