After everything you said, three potential problems come to mind.
1. Your piston is out of spec (too large, not properly surfaced, out of round, etc.)
2. The internals of your gas block are improperly cut so the piston is sitting at an angle to the recess in the slide.
3. Your receiver is improperly cast and the magazine sits ever so slightly too high causing excess drag on the bolt.
These all seem to me to be possible culprits, but im no armorer or gunsmith, so take it with a grain of salt
Yut-either you know what it means or you don't