Trust those site, shoot about 15 feet from target. Align your sites right where you expect the bullet to hit and pull the trigger slowly, had the same problem until I figured it all out. But 90 percent of the glocks are pretty much dead accurate
It sounds like you and the Glock trigger are not in sinc. It may be that they just do not fit your hand which makes for a bad hold and bad trigger control. The G36 single stack is the only Glock that fits my hand and I do well with it. I have short fingers and the thinner grip helps. I do much better with the XD's, Ruger SR's and most other guns.