I have had my dpms ar15 for about 8 years now last time I took it out it fires about 3 rounds then the bolt will close and not chamber the next round. I have tried different mags different ammo all with the same results. My first problem was it was dropping the case about half way out after firing causing a jam. I bought a new extractor spring and it seems to have fixed that but now I'm stumped on not chambering the next round... Any ideas?