This is unusual. The Road Map type 'always displays' You say that the Aerial and Terrain maps display properly for you. The Aerial and Terrain maps do not usually display at zoom values of 8 and higher because the maps are not available at the higher magnifications everywhere as @gordonlu310 mentioned. Try a zoom of 5. 
Please post an example of how your app displays a Road, an Aerial and a Terrain map so we can determine what might be the problem. Thank you.
The Map component uses the OpenStreetMap map tile set. It does not use the Google Map Tiles.
This is an OpenStreetMap of Bejing .
All the roads displayed here should also display using the Map component. If a Road you expect to see does not display it is because it is either not available or the information is blocked.
Part of the same map centered at 39.9190,116.5454 shown on an app:
If I use Aerial or Terrain, there are no map tiles to display. You can see Roads but not Aerial or Terrain. 
On an emulator the Aerial and Terrain do not display however they both display on a real device at a zoom of 5.