Is the Glock too big for your hand? They are too big for mine. From "The Gun Digest Book of Combat Handgunnery" 6th ed, page 51, by Massad Ayoob:
"When the gun doesn't fit and the finger can barely reach the trigger, it will tend to pull the whole gun inboard. That is, a right-handed shooter will tend to pull the shot to the left."
I also found that the recoil from 50 rounds of 9 mm bashed up my thumb, as the grip was too big for me to get my hand around, and the backstrap was hitting my thumb, a classic "h-grip."
Fortunately, I rented the Glock as I wanted to experience the wonder of Glocks before buying one. Nice gun. Small hands. No good.