The only thing I can think of would be to make sure your expander rod (The rod in the sizing/depriming die) is exactly 311 or 312 in diameter, It should at least be the same diameter as your bullet. Providing your brass is new. If not new:
I would also mike your bullets to make sure they are not oversized.
Next, make sure your brass is not to thick at the neck. May have to either outside neck turn or ream inside with 311 reamer.
If none of the above, then you have a sizing die problem.
One last thing, If you are shooting a milsurp 303, the chambers are know to be inconsistent. I would neck size only.