Are you using a full length (FL) die and have it screwed down to where the bottom of the die cams on the top of the shell holder? And are you sure you have the right die? It's not a 300 Win Mag or something else is it? There are lots of calibers that start with .300 . . . .

How much bigger is the back of the resized case than on an unfired case. And, comparing it to a fired case; does the back of the case get bigger after you resize it?