We can see AI traffic alright...Which kind of traffic are you using ? If you are using FS9 models in FSX, have you tried turning Airplane shadows off to see if they reappear ?
Also, the missing wheels, do not have anything to do with the scenery, they might have disappeared because you used the SmallPartRejectRadius tweak in the FSX.CFG, which is a performance tweak that it's usually suggested on forum to improve AI performances. It's not related in any way with our sceneries, and we don't change it at all but, you can set it easily with the Addon Manager interface.
If the wheels are missing only from a distance, instead, it might just be LOD in action.