I’ve had this problem before. I was lowering the resizing die too much and causing the case to bulge. Try rechambering an empty case you’ve already fired to make sure it still chambers and then resize the case that is known to fit. If it doesn’t chamber, obviously need to adjust your sizing die.
Had the same issue one time doing load development for a friend's .30-378. When I seated bullets "by feel", all was well. When my buddy took over seating bullets, he used the "farmer tight" method, and the cases would bulge slightly at the shoulder. Backed off the seating die 1/2 turn and all was fine...
