All vehicles starts adjacent to the terminal and they will go to the airplane. Why they dont use the path?
As I've said:
For vehicles that comes from far away (Catering, Follow Me and Fuel Truck), since they follow the AFCAD, you can set their route by editing the AFCAD. They take the "vehicles" paths by preference so, if your scenery AFCAD has them, it will usually work better, since it minimize the chances of conflicts with other airplanes.
Only "vehicles that comes from far away", takes the AFCAD paths. Of course, the AFCAD paths must be correct, which means your vehicles path should be connected to the other nodes. If the vehicle path is made in a way the vehicle can't reach the parking spot (meaning is not connected to it, or it's connected to a point that will force the vehicle to take a way too long path), they will discard the vehicle path and take the taxiways.
Vehicles that starts at the parking, which are all the remaining ones, DO NOT follow the AFCAD, the go from their starting position on the parking directly to the airplane door, without using the AFCAD at all.
This should be plenty obvious: how they could possibly follow an AFCAD path, since the AFCAD doesn't know anything about their starting positions, just parking nodes (and they are ALREADY starting on a parking!), and their destination (the airplane door) is not fixed ??
So, as I've said, the only control you have over the path that "Vehicles that starts at the parking" take, is their starting position and their starting heading.