You might have a conflicting scenery for that airport, so GSX doesn't know you are running a 3rd party scenery with its own jetways, and thinks you are using e default airport which should have replaced jetways. The most common cause of conflicts are those AFCAD supplied with AI traffic products, which are supposed to be used only if you use the default scenery, but must be removed if you have an addon.
You can use the free Simple Airport Scanner utility to search for duplicates AFCAD:
http://www.scruffyduck.org/simple-airport-scanner/4584282795Or check the name of the AFCAD used by GSX in the parking customization page.