An unfilled impression represents an ad request that didn't return a line item and means that no line item was eligible to serve.
You might think of unfilled impressions as a red alert; they represent lost revenue that could be captured with paying campaigns, and should be corrected as soon as possible.
To find where unfilled impressions are serving:
In your DoubleClick for Publishers account, click the Reports tab.
Select the Inventory report.
Click the edit report link next to the report name, then select Ad unit and Date from the drop-down menus.
Click Columns, then select Impressions and Unfilled impressions from the "Total" heading.
Use the date picker to select a date range and click Done.
Click Run report.
To view unfilled impressions for ad units of all levels, select the Hierarchical view just above the report graph to display each parent ad unit with its child units indented immediately below it.
After reviewing the report, make an educated guess about the location of the tags serving the most unfilled impressions.
Determine whether there are any on- or behind-schedule line items eligible to serve to the tags serving unfilled impressions.
If you're using Legacy-style tags, find the ad tag in question using the instructions in Find Legacy DFP tags on your page, then use TagShooter to determine what line items are eligible to serve to those tags.
If you're using the Google Publisher Tag, use the built-in debugging console to determine whether there are any line items eligible to serve to the ad slots on your page.
If there are no eligible line items to serve to the ad slots in question, you've found where unfilled impressions are serving. To stop unfilled impressions, either eliminate the ad slot from the webpage or traffic remnant or house inventory to the criteria in the tag.
You can proactively monitor unfilled impressions in your network by saving and scheduling the report above.