No admob ad impressions appear on iOS game even though many requests are made

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What steps will reproduce the problem?
1. Integrate adwhirl with an iOS Unity project to aggregate admob and iAd ads.
2. Run the project on an iOS device and wait for ads to appear.
3.

What is the expected output? What do you see instead?
I expect to see admob ads appear in the iOS game but none appear. The Xcode 
console just says 'adwhirlDidNotReceiveAd'. 

What version of the product are you using? On what operating system?
Not sure which version as we use a 3rd party plug-in provider (prime31) for 
adwhirl which we use in Unity 3.4.0.

Please provide any additional information below.
iOS ads do appear but no admob ads. In my admob portal I can see several 
thousand requests across our games on iPhone (Accelerate X, Ascend X, and Aim 
X) but the fill rate is 0% (no ads appear). This is strange as our first two 
games (go chick go, and Chicks Revenge) are receiving impressions no problem.

Please could someone shed some light on this as our main revenue is made 
through admob ads on iOS.

Thanks in advance,
Martin

Original issue reported on code.google.com by mar...@cobaltjungle.com on 14 Feb 2012 at 7:01

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by the way, my email is martin@cobaltplay.com incase anyone would like to get 
in touch with me about the issue directly.

Thanks
Martin

Original comment by mar...@cobaltjungle.com on 14 Feb 2012 at 7:02

[GoogleCodeExporter]

i have the same problem! 
I used prime31 adwhirl plugin and im getting test iAd baner as well but did not 
get Admob baner.

Original comment by buntarga...@gmail.com on 20 Jun 2012 at 12:58