My guess is that it is not setting the shoulder back. When you full length size, the case becomes longer as its sized and then the shoulder needs to be set back to accommodate the growth in length. If the die is made on the long side of tolerance and your chamber is on the short side it could be causing an interference. I assume you are setting the die to hit the top of the shell holder when you size. If not you need to set it deeper until you can chamber a sized case. If its hitting the shell holder already you need to call Lee and get a new die. Hope this makes sense. KN