It sounds like the on/off pin is stuck, not allowing you to pull the valve assembly out of the mag body. You should be able to push the pin in by pulling the trigger and releasing it. If not, remove the grip frame from the rail and that should free the the body and valve assembly. Now, if you still can't pull the valve from the body, or push the on/off pin in to free the valve from the body, at least you can pull the on/off pin out so that the valve will come out.
If the on/off pin was stuck, check for dirt in the on/off when you have the assembly out. Make sure the pin is put back in the proper orientation. It won't work if it is put in backwards.
Except for the Automag in front, its usually the man behind the equipment that counts.