I tested adMob with iOS simulator. It shows ads at the bottom of the screen. But when I change it to real device the ads not showing.
AppDelegate.m
[GADMobileAds configureWithApplicationID:@"ca-app-pub-7067850045620785~8479598780"];
ViewController.m
self.bannerView = [[GADBannerView alloc] initWithAdSize:kGADAdSizeSmartBannerPortrait];
self.bannerView.adUnitID = @"ca-app-pub-7067850045620785/6404933932";
self.bannerView.rootViewController = self;
[self.bannerView loadRequest:[GADRequest request]];
self.bannerView.delegate = self;
I've found that once you've switched to a live device with a non-test adUnitId it can take roughly 30 minutes to build the inventory up to load.
I've experienced this before and found my answer here: AdMob's Help
If the test ad is working, give it some time.
And, obviously, make sure you have access to the internet.