It could be headspace, extractor, rollers, springs, or at worst a cracked trunnion . Check extractor with a dummy round to see of it is sticking. Take the bolt apart and clean it and clean the rifle everywhere. Use oil not grease (its not a tractor like the m1a) Might need different rollers depending on headspace.
If you take it apart including the bolt and clean everything real well to make sure the extractor is not sticking and it still has problems it may need different rollers which you CAN switch out yourself if you can check headspace.
If all is cleaned up and extractor isn't sticking and you cant headspace because you don't feel like it or don't want to go buy go-no-go gages then call up PTR (they have great customer support) Ask for Chris (hopefully he still works there) and let him know what's going on and what you have done to try to remedy this such as checking extractor and such. He will get you up and going. And if all else fails he will have your buddy send it in and he will fix it.
Sorry for your troubles. These rifles usually are very reliable and roll like thunder but as usual with most things there are some lemons out there.
PS also use a GI mag
