Compare the two bolt carriers. I bet the one that works is a "full circle" and the one that came with the gun isn't. It may be counter-intuitive, but a heavier bolt carrier will retard the bolt opening, so the chamber pressure is lower when the bolt starts to open. High pressure keeps the brass tight against the chamber walls, and the result is that the action is retarded. Sometimes you'll rip the rims off the cases with this situation, too.
Let me find a photo of what I am talking about....here ya go:
hth