I see this type of problem all the time at our indoor range. When a customer complains that their handgun is "off", I usually go to the firing point, make sure it is unloaded, hold it up to my ear like it is a telephone (aware of muzzle safety) and after a few seconds, tell the customer that the "pistol says it's YOU, not ME".
Sort of the "Gun Whisperer"! Then I usually take one round, load it up, ask them where they want me to put it and "bang", show them it is NOT their gun.
The BIGGEST problem is that most shooters, including "experienced" (but infrequent) shooters, treat their handgun like a lemon, squeezing the whole grip rather than just the trigger.
Dry firing at a mirror is one of the best things one can do to develop trigger skills. On many handguns, you may want to use dummy/practice rounds, but on Glocks and some other brands, dry firing is OK. Yes, it is a pain to cycle the action each time to reset the action on Glocks and some other similar action pistols. But, consider it training for clearing a jam or other cycling malfunction.
One can NOT get too much practice!