Remove your resizing die from the shell plate.
Disassemble and clean. Inspect the insides with a strong light after you clean.
Place back in the shell plate until it touches, then only a 1/4 more rotation.
Take a once fired brass. Lube it outside, and inside the neck. Resize it. See if it chambers.
Your chamber is a series of two cones (body and shoulder) that feed into a cylinder (the neck area) which feeds into another cone (the throat). Since you are getting resistance at the very last part of chambering that tells me that you are getting interference either at the shoulder or the front of the chamber. Since factory ammo feeds, it is unlikely the interference is in the neck region.
This means that either your dies are out of spec for the shoulder, or they have a buildup of something that is pushing the shoulder out of spec.
Machine guns are awesome until you have to carry one.