You should be able to see ads on your device. Most likely there are no ads available for your device/region.
You can confirm this by looking at the output of logcat. Revmob prints some useful logging info that can be of help.
My experience is that many ad networks (regardless of who they are) have much better performance on iOS than Android.
Side note:
Revmob with their latest SDK have recently changed many of their fullscreen graphics from just using the “Download free game now!” graphics to using an image for the game/app being advertised (similar to Chartboost).