Check to see if your chamber is rough. It should be polished to a 600 grit finish. If it rough and you are generating just enough pressure, even though you may be well within SAAMI specs, the cases could be grabbing the scraches in the chamber and not letting go.

Color the bullets with a black magic marker and see if you are touching the rifeling. This along with a rough chamber could give you just enough pressure to get a case stuck.