There are occasions when the iAd Network will be unavailable or unable to serve an advert to your application. Ads may be unavailable in your app for many reasons, including:
The user having no active Internet connection
The iAd network not currently being available in the country of some of your users
No available banner and ad creatives in the iAd inventory being targeted for a user or your app's demographic
In these circumstances, no banners or ads will appear in your app. If advertising is the key income for your application, you'll want to provide a fallback advert or revenue stream.