Android Question AdColony always returns OnRequestNotFilled


Hi, I setup a project which uses adcolony to display ads. I use Pendrush's library to achieve this. My code is the same as in his example. It worked a few times, but now whenever I try and get an ad the OnRequestNotFilled event is always fired and an ad is never returned. Is there a common reason for an ad request to fail? If so, is there a fix for it? Thanks for any help.