Why does AdMob return NO FILL

To answer the question:

NO_FILL is returned from AdMob when there is no inventory left to serve in the AdMob/DFP backend.

First of all this meant I was requesting an ad of size AxB, but the backend had no ads of this size left to show. It is important that the size you request has inventory left in the backend systems of AdMob/DFP.

Second, their API states that the first time you request an ad to a specific ad unit, you should expect up to two minutes before the ads start serving. I don't know if it is because I am not stationed in the US, but these two minutes often becomes at least 20 minutes if not several hours for me.

I get a 'No Fill' response when I make a request for ads in the Test or Non-Test Mode. What should I do?

In both the Test and Non-Test modes, depending on various parameters such as server load, non-availability of targeted ads, and so on, the Ad Server may send a No Fill response. Try reloading the ad after some time to keep receiving the ads. Although a No Fill is common in the advertising space, you can write to us with your queries.

http://developer.inmobi.com/wiki/index.php?title=Android