That seems to be a problem caused by an AFCAD conflict, the most common offenders are AFCAD files coming with AI Traffic packages like MyTraffic, Traffic X, etc.
If you have a 3rd party airport that comes with its own AFCAD, it's best if you remove the AFCAD files included with these packages for the same airport, that are supposed to be used with the default scenery only.
And/or, be sure the priority of the scenery area that contains the airport you use it's on an higher layer than the AFCAD installed by these products.
You can check which is the AFCAD in use by GSX, by opening the "GSX - Customize airport positions" menu in GSX, the name of the AFCAD used will be displayed in the top left of the scenery customization dialog.