LA:

I had the problem you describe. I spoke (& typed) with dozens of reloaders and got responses from " bad dies" to " case bulge from a compressed load" . What ended up being the problem was that I had my seating die just a touch off. The COL was still in spec, I did the full resize, et al, but the improperly positioned die caused my cases to " bulge" .008 at the shoulder and would not chamber.

Hope that helps.