@Delfin: Mapfactor doesn't "know" what the optimal route is. It simply calculates a route via the A* algorithm, meaning from departure and destination towards each other, cutting off paths sooner or later via the heuristic coefficient (and a few other parameters).
In this algorithm, moving from both destination and departure, calculation is done via "shortest paths", thereby following several paths. In one or more of these paths working towards each other, there is suddenly a "white spot without roads" being Montenegro. That is why it fails.
With Montenegro in place it can follow one of the "best low cost" paths it has already pre-calculated. And probably after having calculated "the cost" through Montenegro it might detect that the route through Macedonia is indeed better (less costs).
And maybe you only think that via Macedonia is more optimal, but theoretical mathematical and statistical calculations prove that through Montenegro is more optimal. And then again: we all know from daily experience that some calculated routes are not always the best because your have 200 traffic lights on that route and maybe one on a 5 km longer route.
So please download Montenegro and test it out. If you really know what the optimal road is you don't need a navigation app.
I use a navigation app for both finding my way to new destinations and, more importantly and the majority of the time, traffic on known routes. I think you may find that a lot of people do the same.
It would be helpful if, instead of saying 'route not found', there was more of an explanation as to where and what the problem is.
For Navigator to find a route from Munich (or Prague, Vienna ..) to Athens, the map of Bulgaria must also be installed.
Navigator then suggests a route via Sofia. This is 212 km longer than the route calculated by other navigation apps (Here We Go, Google Maps, OsmAnd ..).