Could be a couple of things.
1. Are you holding the lever in the "unload" position for 2-3 seconds?
2. Can you verify that the unload function on the operating track is working?
3. There is a microswitch that i used to "cut" the power during its operation. Take the shell off to make sure that a wire hasn't come un done. Then I would check the switch to make sure that its functioning.