Sometimes you have to wait... Patience can be the name of the game with this. An Ad unit sometimes takes a little time to start sending and sometimes it is a bit fussy if not on WiFi
1) Is it a real device?
2) Have you checked the Admob Control Panel - it usually indicates if the Ad is active, etc...
?
ETA : I think the modules are Ad_ReceiveAd and Ad_FailedoReceiveAd - check that you have both of those and put some logging in or toasmessage to see if they are being fired.