That's because that airplane is not internally supported by GSX, so you must create a (correct) GSX configuration for it, using the integrated airplane configuration editor.
If you HAVE made a configuration for it, the lack of a vehicle usually means a wrong door configuration: either you set the height wrong (too low, too high), or you set the correct placement but for the wrong door (for example, putting the arrow pointer for the service door in front of the cargo door )
