Another thing to check is the slide rails on the frame. Micro-cracks can cause extra flexing during use and allow the slide to move too far back. Of all the Glock models, those in .40 S&W seem most effected by micro-cracks in the rails, especially on older guns that have not had the recoil spring assembly changed at the recommended (2500-3000 rds) interval. It's that sharp/snappy recoil of the .40.