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.