Trigger finger placement. I had to work through the same problem on my Glock 34. The problem for me was the part of my finger I was using to pull the trigger. When I used the pad near the tip of the finger my impacts would be low left out of the A zone consistently while doing Bill drills. The solution for me was putting more finger on the trigger (it is almost to the knuckle now) and now my hits are As. You should also be able to see the problem during dry fire using the drill "trigger control at speed" (I found it in one of Ben Stoeger's book) pay close attention to your front sight and you will see it dipping low left. Experiment with placement until the front sight doesn't move when pulling the trigger fast.
Drill - aim at BLANK WALL not at a target, focus on front sight, at buzzer press trigger quick as possible