Is your die set up correctly?

With lee dies at least if you set the die so it just touches the case holder then try and close on a fired case you will generally find that the case will not FL size and there will be a gap between the holder and bottom of the die. If you then screw in the die the further 1.5 (or thereabouts) turns stated in the instructions the case will FL size and you'll be able to push the case holder right up to the bottom of the die.