MacStories posted a message from one App Store developer who recently saw the iAd fill rate for his Dex application for iOS drop to zero. He asked Apple and received the following answer:
We periodically review the apps in the iAd Network to ensure that all apps receiving ads are aligned with the needs of our advertisers. Currently, our advertisers prefer that their advertising not appear in applications that are targeted for users that are young children, since their products are not targeted at that audience.