iAds will show test ads until Apple approve the application. Once the application is approved, the iAds will be switched to live.
You didn't mention your location...
As far as I'm aware, iAds are still only showing for users that live in the USA. Even then, the fill rate for iAds have been shown to be around 10-11% (not very high at all).
This means that for every 100 ad requests received, only 10 or 11 are going to actually show an iAd. the other 90-89% will show nothing.
Hopefully the fill rate will start to improve as Apple's ad inventory increases in size.
The other issue is that it seems the iAd framework will not show repeat adverts. That is to say that once a particular iAd has been shown, it won't be shown again on that device for that session (the length of a session is vague too...).
So, check your location, check your current fill rate on the iTunes Connect iAd section.