I would guess that the mold is bored off center. What I am trying to say is that one side of your mold is holding the bullets because the cavity is offset more to one side that the other. If you have a way of measuring your cavities you will find that one side will have a deeper lateral depth than the other. I just ran into this with a Lee six cavity Harris mold. This mold was bored .010 to .015 deeper laterally on one side than the other and held all of the bullets in that side requiring "beating" on the hinge bolt to release them. Lee was very good about replacing the mold and the new one is perfect.