Wrong forum try in the reloading section.
Could be brass variation or different amounts of spring back. Typically I polish the mandrels down for about .002-.003" of neck tension' the factory mandrel can be .001".. You can order smaller mandrels from Lee. Might try annealing the cases as this will never hurt anything. Reaming or neck turning the brass could help but you would then have even less neck tension.
Just on the off chance you have some funky bullets check their diameter. Some of the +P Roberts brass have thick necks.
