If you decapping pin slips excessively, remove the stem, de-grease with brake clean (or acetone, lacquer thinner, or other aggressive de-greaser). Also de-grease the collet. Reassemble and tighten the collet pretty tight. But be aware if the stem doesn't slip there is a chance the pin will break if a solid item is struck during decapping (off center pin or flash hole, or a Berdan primed case mistakenly slipped through). I've found a "happy medium" for how tight to tighten my Lee die's decapping stems...