Strange. I released Monster Attack just last week and it does work for both networks.
-Construct R148
-CocoonJS latest version (For android it is 1.4.4)
-Admob and Millenial Media.
Mind posting how you call the ads ? Try add more than one networks ads.
How about ads refreshed ? Did you set it in mopub ?
There is alot of possibilities about what could be wrong or missing.