Could be the ammo , try another load.

3 things to try if not the ammo
Make sure the extractor dose not have crud behind it built up on bolt face

Take the barrel off and look if the chamber is dented in by lots of dry fireing , lightly hone the chamber dent out

Replace the Mag. I had a mag that caused extraction problems ( don't know how ,with 1 of the 3 mags I had it jammed , but after I replaced it problems went away )