Hi all!
All what is written down is pure nonsense!! You realy do not know where you are talking about. You all just guessing.
I have the same problem at SOME airports. I use Amsterdam Schiphol and I cant get it working there whatever I try! It seems these jetways simply do not work 
So what I did to check is let the plane as it is on the gate on Schiphol and simply go to world/go to airport. (By the way I use the Boeing 737-800). I choose London Heathrow, Gate 105 Gate Small. Just hit ctrl+J and yes it works like a charm. EVEN when I turn Scenery LOW and advanced animations OFF!!! The only thing that needs to be more or less HIGH are: the scenery complexity and Autogen density. When these two are low, the jetway will not be present. As soon as you see the jetway it will WORK, no matter what all other settings are doing!
So when it does not work, the airport scenery is simply not ready to get the jetway working. As an example, the people behind the windows in your airport scenery are also not working around. They are not animated. The jetway should be animated during the process of making a scenery.
Hope this will make more sense than just guessing.
Have a nice day and a good airport scenery
Ed