OK, if you're going to spend any time taking bolts apart, go to Brownells and buy the bolt disassembly fixture.
http://www.brownells.com/gunsmith-to...ake_3=AR-15/M4
Use a proper-sized punch, and remove the pin, ejector and spring.
What you'll most likely find is the tunnel is packed with brass shavings. These are binding the ejector. The forward rim of the ejector is jsut a bit too sharp, and it is shaving bits of brass off the case head.
Chuck the ejector in a drill, and use a stone to just knock the sharp edge off the ejector, no more. You do not want to bevel or round it.
Scrub, reassemble, and get back to shooting.
We see this on a fairly regular basis in the classes, and this has solved the problem in all instances but one.