Lock the bolt in it's open position. Check the ejector (round pin on left side of bolt face) for free movement with a non marring tool. It should be able to be pushed into the bolt body freely, but with some effort. If not, try lubing it with a good grade of light machine oil.....
I once had an ejector fail on an older BAR due the above circumstance. I assume that you've checked the gas piston for free movement, haven't you?