Maybe a long shot, but I had the same issue with my tracker 12 ga. Mine was tapped for a scope and it wasn't tapped blind so the holes were tapped through and the scope base screws weren't filling up the holes so the brass and the plastic on the empty shell would flow up into the holes when shot and hold the empty in the chamber. I filled them with JB weld and radiused them to the chamber and all is good now. If yours is tapped from the factory, it "should" be blind tapped but it's worth the check. Check the spent cases for abnormalities....