There was an old post about this but I've never been able to find it. At high zoom levels (20 and 21), the Aerial mode usually doesn't show images. Other modes show a zoomed-in version of the previous images, including AerialOnDemand. There doesn't seem to be a way not show labels with AerialOnDemand, unfortunately. Can Aerial be changed so that it also shows the zoomed-in previous images?
Bing documentation says: "Note: Some imagery may not be available at all zoom levels for all locations. If imagery is not available at a location, a message is returned in the ErrorDetails collection of the response."