IMHO the RCBS Universal Hand Primer is far, far superior to any other hand priming system I have ever used. Absolutely seats every size primer (every size, in Winchester, Federal & CCI brands) I have ever attempted to seat, albeit in properly cleaned and sized cases. Kept clean and very lightly lubricated (jaws only) the primers are seated with very little effort. Easy (at least in my experience) to "feel" the primer being properly set.

I'd be checking out the primer pockets before I'd blame the RCBS unit. Only thing I can imagine is staked or crimped mil-surplus brass, perhaps steel, brass-plated or aluminum (non-reloadable?) cases...

Just my personal experience: I would no more prime on the press than use an empty shell case to measure powder without a scale.

Cheers!