A fat bodied case with a sharp shoulder is what gives.
The WSM's have this problem quite regularly.
As you size the neck of the case you actually start the base of the shoulder to bulge out slightly, thus causing your tough chambering. I had to remove a couple of thousands of an inch off the bottom of the sizer die to get everything to size up just right.
I originally removed a bit of material from the shell holder and it worked fine till I ripped the shell holder to pieces on a tough to extract(poorly lubed) case.
I used a belt sander to remove the material from the bottom of the sizing die, just go slow keep a cool tool, and remember to wash the die out with brake cleaner when your done.
