ArcGIS 10.2 "Drive time analysis" service area problem

Although reasonably experienced with ArcGIS I am a bit of a newbie with Network analyst. I have a situation in which things work Ok with the tutorial data but not with mine!

I am trying to determine an 8km road distance from a set of hospitals in East Africa. I have some roads data that I can build into a network. As far as I can see there is full connectivity. If I work using detailed polygons (which is what I want) I get this.

So the "service area does not go along roads A,B and C, which I would expect it to. and how on earth does it think I am going to get to Point D!!.

Using generalised polygons I get this:

Now I have lost the slightly anomalous point D, but the services area still does not traverse roads A and B

Any help gratefully appreciated

Rob Corner. Western Australia