"Does this sound like a minimum size brass issue or a maximum size chamber issue or combination of both?"
I believe you have named your problem. The .375 headspaces on the belt, and if the brass is too short in the neck area to fill the shoulder, the brass will stretch between the belt and neck, causing the problem you are having.
As you mentioned, it could be brass that is too short, or a chamber that was reamed too deep. Or both.
I had a .300 Wby. that did that. Cases fired in this rifle, even after FL resizing, would not chamber in another .300 Wby. I had.
