I have battled for a long time with the problem that sometimes AI controlled vehicles will not move and follow their waypoints. But there was no clearly identifyable reason for this so far, sometimes they did not move from the beginning, sometimes suddenly during their route. It also seemed to be random, as it worked most of the time.
A common observation was that it seemed to be always at a position when getPos _vehicle returned a z position < 0, sometimes very small like 1E-006 or something similiar.
Some more recent testing trying to identify the issue showed that the negative z-position seem to be the issue, it looks like it is not detected as a valid position or something like that. So the cases where this problem arrives is mainly if the vehicle is spawned at the road at a position where z < 0 or if the vehicle stops after arriving a waypoint, to calc the route to next waypoint and by chance did that on a z < 0 position,
Vehicles however will drive over these z < 0 positions just fine as long they do not have to stop there.
If i do a doMove to the group leader of the vehicle to a different position 1 or 2 meters away, the vehicle will start moving again, first to the doMove position and then keep following the waypoints again.
At least the recent tests also showed that the AI has also deactivated the engine (isEngineOn = false) when stuck on the position.