Jamming how? Doesn't extract. Doesn't feed. Double feed. It only works when a mag is inserted with the bolt closed(In battery). And the bolt is racked to load the chamber? Verses inserting the mag on an open bolt(out of battery). Releasing the bolt and it "jams".
We need details.
What happens if you cycle a few rounds with the charging handle?
A couple of things: Could be a damaged extractor or weak spring. Possibly a damaged ejector pin or weak spring.