Ads service not working on android build

I have just finished developing a game and started working on unity's ads service integration (through the service window inside unity itself). It seemed pretty straight forward, and I have been able to make it work in the editor but when I try and build the game, it won't show any ad. It's kinda odd. The Advertisment.IsReady() function always returns false. The project is made in Unity 5.3.5f1 and I have also tried building it with Unity 5.4.2f2, on an Nvidia Shield tablet, LG G3 mobile but I always get the same result. Does anybody know what may be causing the issue??