One or more of your ad units do not have matching div-IDs. There’s possibly a typo error in the ad unit name. As a result, DFP could not attribute the impressions. Those ad units are receiving a high proportion of unfilled ad impressions which are labeled as “Default Ad Unit”. To prevent this ad revenue leak, targeting and on-page setup adjustments must be made.
Step 1: Audit the ad units on the page and correct any spelling errors. Open the website > Right click > View Page Source
Step 2: Ad Unit names should match the ad unit code on DFP