In the past I've had the same problem and got it resolved by launching my application multiple times. Once your application has made a number of requests ads get fed to it. Not sure why this is but it is what I've seen before. I'd suggest loading the app in a simulator then launch it, let it run for a minute and then close it. Do that a few dozen times and eventually you should start to see ads pop up.
"Not to know what happened before you were born is to be forever a child." Marcus Tullius Cicero
