Google recently launched a full release of the Content Ads Diagnostic Tool  (CADT), which is designed to provide greater transparency to Display campaigns and placement of ads across the Google Display Network. With CADT, Google now provides specific reasons why a specific Display ad is not running.
First step: Go to the Networks tab and Show details for managed placements.
Second step: Hover over the speech bubble icon under the Status column and the diagnostic info will appear, similar to viewing a keyword’s Quality Score diagnosis.
Note: CADT can only diagnose managed placements and topics (even more reason to add automatic placements as managed placements on an ongoing basis). Google plans to support re-marketing and interest categories in the future.
How to Run a Placement Diagnosis:
My initial excitement over this new tool soon fizzled when, upon review of one of my accounts, it was discovered that out of the top 30 performing managed placements, almost 85% of the ads were ineligible for display due to one of the reasons listed below. And out of the 9 diagnoses Google provided, only 3 linked to AdWords help articles on how to improve performance.
9 reasons your Display ads aren’t running:
1) Your ad is not showing on this placement because it has lost in the auction to more competitive ads.
2) Your ad shares a similar Display URL with a competing, higher ranking ad.
3) Based on the targeting keywords you have specified, this page is not relevant enough for your adgroup.
4) Your bid is too low for your ads to show.
5) Your ad is not being displayed as your ad is not supported by the publisher.
6) The combination of your current campaign settings and targeting criteria do not appear to have enough Google AdSense pageviews to be meaningfully diagnosed.
7) Your ads are currently set to appear to an audience that may not understand the language of the creatives.
8) This ad is not receiving all the available impressions because your quality score is below our threshold.
9) This adgroup does not appear to be available for serving. Please try again later.
Google’s new Display tool has the potential to be a very powerful tool for marketers, considering how difficult it is to keep track of where your ads are and aren’t running when dealing with a large number of sites, but it still needs fine tuning in providing actionable account-specific steps to improving performance of your ad.