If only some objects are missing or parts are missing, it's usually a problem of video memory corruption so, you might try all all these:
- Reset your video driver to default
- If that doesn't work, try to delete your FSX.CFG, so a new one will be created with all default settings. Apply only the HIGHMEMFIX = 1 tweak and nothing else, if you use FSX.
- If that doesn't work, be sure you are not running the sim or any of the addons that starts with it in Compatibility mode.
After resetting the FSX.CFG, you must reinstall the scenery and GSX too, otherwise it won't find its own Simobjects, because the FSX.CFG must be edited by the installer.
