are you taking the brass up into the die far enough - camming over fully?
Do you feel resistance as you lower the brass out of the die after sizing? if there is no resistance, then the die is not sizing the neck properly - maybe not camming over or just a bad die??. If you do feel resistance, then the ball is likely too big and making the opening too large.
(When sizing, first the sizing part of the die makes the neck smaller, then the ball expands it out to hopefully the proper diameter)
