Be sure you use the default configuration that comes with GSX, using the "Reset" button in the airplane configuration editor, in order to be sure it's not a problem caused by having edited the config.
If still doesn't work after a Reset, check you don't have any GSX.CFG file in the airplane folder itself (a file located here won't be removed by the Reset button).
If still doesn't work after you are 100% the configuration used is the internal one (the airplane configuration editor will tell you that), please indicate a precise reproducible case of which airport you are in, and which parking. Possibly, a default or an FSDT airport.