Riser?
Are you using a riser with the AutoDisk? If so, it has its own interal floating "plug" that you can accidentally let fall out.
If that is not the problem, here is how I would go about sorting it out:
First, remove the Auto Disk from the die and any riser.
Verify that the charge disk is firmly in place with no back and forth play - this will show that the actuating lever is in the correct slot under the disk (very easy to get that wrong when replacing a disk).
Next, remove the spring from the actuating lever. This will allow you to easily slide the collar (where the threads are) up and down on the drop tube to verify that the charge disk is moving properly. Measure how far the collar has to move down on the drop tube to make the charge disk move over the center of the drop hole. Be cautious you do not move it too far or the disk will actually move all the way out of the slot - no problem really, except that you just have to be careful to get the actuating lever back in the right slot when you re-insert the disk.
If it is moving correctly, go to the expander die without the Auto Disk installed and insert an empty shell case. Use any rod or medium sized screwdriver to stick into the top of the expander die and rest it on the expander plug. As the shell case reaches the top of the expander die, measure how far the rod is moving up - it must be the same amount that you measured in the step above.
Repeat that step with a riser, if you are using one. Movement must again be the same amount.
Now put the Auto Disk back on the expander die (still with the spring off), then re-insert the shell case and the disk should move the correct amount.
If the disk is installed correctly, these test WILL show you where the problem is.
When all done, make sure you put the spring back on!