There's a motor for this door also. Not to be confused with the motor which makes the Death Star turn. Does the door open at other times? When set to Factory It should open on the third ball if you haven't opened it earlier by advancing the moon. Is it doing this?
When I first brought my SW it was a dog of a machine with every problem imaginable. Great game now but I did end up having to buy a new motor for the Death Star and R2-D2. May not be your problem but I just thought I'd mention it. The motors were very noisy and only worked occasionally.
