If at any point a vehicle in a convoy gets too close to the vehicle ahead of it, the vehicle will halt permanently. Moving the vehicle in Zeus causes it to return to the convoy starting position, and then retrace the entire convoy path.
This is especially noticeable with convoys that have the "full" speed mode and "file" formation. Additionally, if the vehicles are placed too tightly to each other, the back vehicles will stay halted forever.
Remote controlling the vehicle and inching it a bit forward seems to fix the problem.