I have had this happen. Take a look at your brass. Some brands have a small radius and some brands such as the old Remington cases have a larger radius.
It seems that when the rims drags back about 5/16" there is a bit of a notch in the top of the receiver magazine opening. About the time the rims hit this notch the cocking piece is also overriding the sear. Both produce a little snag.


Quote Originally Posted by Parashooter View Post
OK, one more possibility. As the bolt is pulled back, the extractor forces the case to the left, where it drags against the receiver wall. (This friction is what normally ejects the empty case - long before the rim could strike the ejector.) If the extractor spring is strong and the wall is rough and/or dry, the resultant friction can retard bolt movement significantly.



Try smearing a little good grease on the wall where the cartridge rim drags. If that makes things better, polishing the site of the friction may help, as may keeping it lubed.