Another thing to look at is "wristing". In this case, when you pull the trigger, you also clench your other fingers tighter, causing the shot to go down and left.
You can see what I mean if you make a loose fist and then watch the inside of your forearm. Increase the pressure of your trigger finger only: Very little movement of muscle in your forearm. Next, tighten the grip of the other three fingers. See the muscles jump?