This is due to the mapping data itself, not apples application. If you pay attention and look closely to the intersections and roundabouts etc. many of them won't 'join' or match correctly. Because of this, the application is unable to work out directions between certain points. To check if this is the case, try getting directions between two simple locations, on the same street. It should work. I'm in Malawi, and notice this same issue.
This is simply down to the quality of the mapping data itself, even google maps had (and still has) issues with directions in Malawi, until about a year ago they did not work at all. The solution then is to improve the data. Unfortunately, as apple gets this data from multiple sources the quality varies significantly. With google maps, one could submit mapping data and corrections, but with the new maps there is no such capability that I am aware of (open maps perhaps?).
It's a difficult thing to solve, but fundamentally it comes down to apple improving their sources for mapping data. For developing countries this could take a long time unfortunately. Hopefully with more partnerships like tomtom this should improve, garmin have excellent maps for Malawi for example. At the moment I suppose all that can be done is to make apple aware of the lacklustre data where applicable. Maybe try contributing to openmaps or the like.
