Google Ad Manager, is Googles’ hosted ad management solution for publishers that help you sell, schedule, deliver, and measure all of your directly-sold and network-based ad inventory. Here’s some tips managing for publishers, trying to figure out why a particular ad isn't showing:

-Google Ad Manager debug tool allows you to easily view the real-time decision making process Ad Manager uses to determine which ad to serve.
-One of the most common reasons for an ad not to show is due to competing line items that have a higher priority to deliver.
-line item delivery tab on the orders page of your Ad Manager account provides a quick and easy way to catch delivery issues before they happen.