Missing imagery when using BingMapsImageryProvider

I am just getting started using Cesium and have been able to have some success so far. However, I am currently stuck with an issue with the BingMapsImageryProvider. I have set up an API key and have specified the url as http://dev.virtualearth.net/ because my application is running on the file:// protocol. I am able to get imagery when I am zoomed far out (e.g. looking at an entire city) and zoomed far in (e.g. looking at individual homes). However, in between there are a few zoom levels where I get no imagery and the background looks like everything is floating in space. Is there a common cause for this issue or ways to diagnose what is going on?

Thanks a lot,

Additionally, I don’t seem to have this issue in the 2D SceneMode, only in the 3D SceneMode. Setting the scene3DOnly of the Viewer does not solve the problem either.