I was always under the impression that the "shoulder" was designed to slide into the threaded part of the switch when the door was closed for the first time. Due to production tolerances, the distance from the door to the door frame will be a little different on each car. The sliding "shoulder" allows the switch to "self adjust" to the car it's installed on. I've got a few switches and the shoulder is pushed in by different amounts on each one.

Maybe the "shoulder" is stuck in the threaded section. You could tap on the end to hopefully compress it a little bit to get it started. Put some tape on the door and try closing it again.