In the old days when you rolled you brass on a pad to lube them, this would happen with a dirty pad. Now days it usually means that either a die is scored, which seldom happens, or is dirty, or the brass is dirty.
I assume when you clean these dies the plugs whether they be re sizer dies, or seating dies are taken out. When I clean mine I usually use lacquer thinner as a solvent for dissolving lube in the die, but then wipe out with an lightly oiled cloth to prevent rust.