If it's just static airplanes and cars, but nothing else is missing, maybe you are too far from the object (because they are switched off after a distance, which I think it's 0.4 nm) or you are too high (there's an altitude switch off as well)
If you are on ground an close to the objects and you still are not seeing them, you can try acting on the Addon Manager Anti-popup slider setting, which will increase the distance range of such objects. Try a setting like 2 or 4.
If it still doens't change, and you see everything else correctly...than I don't have any idea how what might be. Uninstalling and reinstalling the scenery might help.