Sometimes trying to apply too much crimp will create this condition. The brass case has temper (or "spring back"), while a cast lead bullet has next to none. When you apply an excessively heavy crimp, the case gets rolled into the bullet, creating a groove in the bullet. After the cartridge is withdrawn from the die, the brass springs back slightly, allowing the bullet to spin in the groove, but the groove also prevents the bullet from being pulled out of or pushed further into the case. The excessive axial load placed on the cartridge case also causes the case below the crimp to bulge, destroying the press fit between the bullet and case formed in the sizing die.
Back the Factory Crimp die out slightly and your problem will go away.
