I think I figured something out after spending a couple hours messing with things.
It seems that when my dome slaving settings are all set to 0.00, the dome will go to 270 degrees and do nothing else. When I set some numbers in there (like the attached photo) the slaving works!
But, for some reason they aren't saving the settings when I shut down the Pi, so when I restart they are all back to zero. At least I'm making some progress with this problem.

Tom