I'm not sure if it's due to some sort of limit on the vehicles that can appear on the compass, but recently I've noticed that trucks I have recently constructed, and occasionally some older ones, are not appearing on the compass. Perhaps I have too many? Perhaps this is an anti-clutter feature, but other trucks and tractors appear from across the map, whereas these ones are not visible on the compass even when I am standing close to them. They do appear on the map screen, however.