Clean all of the oil out of the cover detent and open and close it for a few days. When we got some of our new M4s and M16A4s there were some issues with the detents that were stiff and caused some feed issues.
Other than that, you might want to talk to Stag and see what they say about it. If worst comes to worst, you might have to replace the ejection port cover.
Never had issues like this with the M1A didja'?