I am going to guess it is a worn extractor tail. What you are probably experiencing is the bolt head wanting to rotate while moving the bolt in or out. The extractor tail resists this rotation. It is a known weakness of this design and one of the reasons extractors are hard to find. I have a link on my home computer that explains it very well. It also has a work around of bending the worn extractor tail to help the little nub engage the recess in the bolt body. Not really a proper repair but may work. I will search for the link when I get home.