Changes in grip pressure cause most missed shots. Anyone who gets low left shots (right handed shooters) or low right shots (left handed shooters), is changing their grip pressure while adding pressure to the trigger. The key is to maintain grip pressure while adding pressure to the trigger. Turn grip pressure on while extending the gun. Keep grip pressure on while firing your shots. Only turn grip pressure off after retracting the gun back to the ready position. Make it a cardinal sin to change grip pressure while the gun is extended. This is a very large topic and I'm sure you don't want an entire book in this post so, the last thing I'll suggest is to do daily reps of isolating your index finger from the other fingers. Call me or better yet, come to a Handgun Science class. I'd be happy to talk with you or show you what to do to develop the proper motor skills mentioned here.
I look forward to shooting with you.