Was it Lake City brass? That only happened to me with 308 Lake City brass.
They sell an RCBS "suck case) "kit" for that very problem.
he doesn't have a stuck case per say. When he began the sizing process he did not have the ram fully up, as a result when he set the case into the open jaws of the case holder they were actually closed. So the case was just sitting on top of the jaws. Now the case is up in the die with no normal means of latching on to the extractor groove and withdrawing the case. Or so me thinks...