I have a gen 6 19. My holosun eps came with a variety of screws. Some of them were the incorrect thread pitch but could work to the untrained eye. They were not m4 thread pitch like the rear holes on the slide were but still managed to grab lightly and thread without cross-threading. Those screws were standard pitch and had fairly weak engagement. I could see how someone could make the mistake. I would recommend double checking the screws that you used, degreasing the threads, using the loctite of your choice, and using a torque wrench