My one suggestion with 7.62x39 AR's is Dont' go cheap on mags. THe best ones are frankenmags. Avoid the tapco cheapies liek the plague. Also, watch the bolt.
Buddy of mine built a 7.62x39 upper. He was plagued with problems. Several problems turn up. The bolt was DPMS. the ejector was inserted backwards. This was done at the factory, as he purchased a complete bolt/carrier from DPMS.
Second problem was shitty mags.
I'd check the extractor. Pull the bolt out and try to stick a round in the face. See if it snaps in place, then come under spring tension (ejector test). IF it snaps into place, but doesn't want to jump out, Your ejector may be messed up.