Ok. If the request fails, you should get a logging message in LogCat with an
error code.
If the code is 0, it's hard to tell what happened since it's an internal error.
If it iss 1, your ad unit ID is probably an invalid format (referenced incorrectly).
If it is 2, it's a network connection error.
If it is 3, then there was no inventory. Also check LogCat for a message that says "To get test ads on this device, call AdRequest.Builder.addTestDevice("012345abcdef");" That will be your test device ID. Add that into your code to see if you can get test ads back.
A common problem I've seen related to error code 3 is your ad unit specified type "Interstitial" when you created it in
apps.admob.com, but you're requesting a banner. Double check the settings of your ad unit on
apps.admob.com. If you see a setting for "Interstitial timeout" but not a setting for "Refresh rate", then you have an interstitial ad unit and that's why you can't get banner ads back with it.
Thanks,
Eric