I'd say you're not resizing enough and you likely have negative headspace. Size a round as you do normally, chamber that brass and place a straight edge across the chamber, the headstamp should be flush with the chamber mouth, if it's too high it won't allow the action to fully lock up. You should also see a slight gap between the standing breech and the barrel that's not there with an empty chamber if you hold the rifle up to a light an look across the barrel to breech joint. You'll have to turn the sizing die into the press a little to push the shoulder back a little to get the brass to fit the chamber and allow the action to close.
Tim