Give the die about another quarter turn down, and then try sizing one of the cases that has no neck tension. Use a bullet to see if it gains any neck tension. Sometimes dies need a bit of cam over to get good neck tension.
If that fails to work then remove the decapping pin/expander. Size the round, then afterwards reinsert the expander, run it to just inside the neck enough to expand it. See if there is enough tension in the neck. If not then contact RCBS, and let them know what your problem is. They will more than likely mail you a new expander button.
It could also be a brass thing if they are all the same head stamp. Some brass has more spring back than others.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.