After getting to the observation deck with Sally, Paulson, Elliot and Somah ALL of the characters just stand there and do nothing, halting the progress through the Mothership.
Can anybody please help??
-Logan

This happens occasionally. If you're on the PC, you can get around the problem by going up to the door, opening the console ( ` ), clicking on the door, typing "unlock", pressing and closing the console ( ` ) again. You can now open the door that Sally was supposed to open for you and you can proceed from there.

If you're on a console, about all you can do is reload a saved game and hope for the best the next go-around.

When I use the unlock command at this stage of the DLC the NPC's seem to remain frozen in this place until I have progressed to final stage of the DLC, then they turn up again. At first I wasn't sure if they were supposed to follow me somewhere else or if I needed Sally still to open any more doors etc, but it was ok.