Sorry Umberto I am afraid you are wrong on this occasion. The buses have reappeared with no changes from myself after some considerable time of disappearing. The only thing that has occurred on my PC is a major windows automatic update so maybe this had an impact I have no idea! 
Which clearly indicates, instead, that I was right...since I've said the problem was some of your settings, it's very likely that a Windows automatic update has reset something in your settings that affected the video card. Might even be something obscure that not many knows about it, like having drivers running under the WDDM driver model 1.0 or 1.1, that might be triggered by something (even not related to FSX) forcing the driver into a Compatibility mode.
The former is a lesser-performing method that your driver might use for compatibility reasons, that we found has an affect on some FSX objects, making them invisible, but a normally installed system (WDDM 1.1 has been introduced many years ago), should always work in WDDM 1.1
So yes, it WAS a "settings problem", as I've said.
They have reappeared but as usual drive through everything in their path including the luggage carts, aircraft and so on! It is the one thing in this programme I find very annoying but I believe it has to do with AFCADs or something like that?
If depends what is driving into, and what you are referring with "luggage carts". If those are static objects that comes with the scenery, as explained too many times already on the forum, BECAUSE GSX can't possibly know anything about the outside world EXCEPT the AFCAD, since those are NOT in the AFCAD, they can't be seen by GSX.
HOWEVER, GSX being the incredibly flexible program that is, most of these issues can be fixed by customizing the starting position of vehicles, to start in a place were they won't end up driving through static scenery objects. SOME later FSDT sceneries are more clever, because they "know" of GSX, so they keep a list of objects that GSX has permission to remove from a parking when you select it for GSX usage, to reduce its ground clutter and minimize conflicts. But this happens only in more recent FSDT sceneries. For other sceneries, you can use the "Customized parking position" option and move the starting position of the vehicles.
We supply all our sceneries with pre-made customizations, but we can't obviously do this for every scenery out there, because we would then have to charge per-airport, which is wrong. Instead, we give you the tools to freely customize the scenery.
About AI airplanes, this is USUALLY a problem of the AFCAD. GSX tries to use the dedicated vehicle paths ( the pink lines in ADE ), even when the path might be longer, in order to minimize potential conflicts with AI airplanes, which is the same reason why such path exists in real life.
HOWEVER, depending how the AFCAD is made, it's possible there are not enough dedicated vehicle paths, or even none at all so, GSX can't just stop there, it WILL use taxiways, if dedicated vehicle paths are missing, and that could create a conflict with AIs.
And that's why the vehicles are INTENTIONALLY made so they won't stop yielding for an AI airplane, because that could create traffic deadlocks very easily, since it's obviously much better to have vehicles passing through AIs, rather than having a ground traffic jam between GSX vehicles and AIs.
Again, this usually doesn't happen if the scenery has proper dedicated vehicle paths, that allow to reach all parkings without going through jetways. And not just dedicated vehicle paths, but also dedicated starting vehicles starting points which, again, GSX will correctly use by preference but, again, if the scenery designer forgot to add them, or placed them too far apart or not connecting properly to the paths, GSX cannot just stop working, but it will than use a regular airplane parking to spawn vehicles that comes from afar (Catering, Fuel Truck and baggage trains when Boarding), which also will increase the chance of conflicts with AIs, since regular airplane parkings are usually connected to the taxiways network, instead of dedicated vehicle parkings that should be connected to the dedicated vehicle paths network, if the scenery AFCAD is made properly.
Basically, it all boils down to the AFCAD being down properly. Trash-in -> Trash-Out
