This one may seem like it came from left field, but Mo what kind of loads are you running? I had a strange experience with .357 mags a while back. I was attempting to dummy down the loads for my wife. She had just started shooting and was real timid about the recoil. I took the load data for cowboy loads and actually dropped them a bit more. (Yes, yes I know that was real stupid...live and learn). The charge was so light that the pressure actually belled the bottom of the case. The cases would pop right out of the cylinder, but would refuse to chamber. Again, not all of them would do this. When I mic'd the outside of the cases, I found that there was a slight buldge (0.05) near the base. I cranked up the powder charge to where it should have been and the problem went away.
Also, I have heard that cases that are past their time can do this also. I have seen cases that develop a ring just above the
just a thought.