Sometimes beads refuse to come off the mandrel with normal twisting of the bead on the mandrel. In these cases I use two methods.
I lock a small vice grip (locking pliers) on the mandrel near the middle and hold bead with non slip material. Then I twist back and forth to loosen the bead on the mandrel. When it frees from the mandrel, I pull toward the end continuing to twist.
If it does not loosen, or come off the end, I soak the whole in water over night and then repeat the above procedure again.
If these two do not work, I sacrifice the bead, as even using soft jawed vices normally leads to the crushing of the bead.