OP has another M96 in standard trim that closes much easier. I would be a bit worried that other mods had been done. Does the bolt number match the receiver? Any signs of grinding on the camming surface? Has striker spring been replaced or adjusted too tight? Clean and lube, yes, but this sounds like more than that. Does it make a difference if there is a round in the chamber? Is the round coming out of the magazine going under the extractor hook OK? These actions are usually pretty slick.