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Are you -sure- the extractor isn't damaged, or has dirt under it? If it doesn't, I'd look long and hard at the profile of the case rims. See if there's a difference in the shape of the extraction groove on the rounds that failed to eject. From what you describe, the extractor is "jumping" back off the rim on the cycle, leaving the spent round stuck in the chamber. A broken/chipped extractor tooth, excessive carbon buildup under the extractor, or improperly shaped extraction grooves on the cases, seems to be the most likely culprits to me.

Another possibility too is a dirty chamber. If you've shot Wolf or other steel-cased ammo recently, you may have some of their nastiness built up in your chamber. I went through this with one of my ARs, and practically had to chuck a brass chamber brush up in my drill to clean out the laquer and crud left by the Wolf ammo (relax guys, I didn't really do that! LOTS of scrubbing by hand, tho').

So, in order, I'd look at the extractor, look for dirt buildup under the extractor, look at your fired cases, and look inside the chamber.