Google Ad Manager (GAM) was found in the header of the page, however, several ad units did not have matching div-IDs. Every misconfigured ad unit will not serve a monetized ad impression and is losing ad revenues on every page view.
See our DFP Tutorial: Best Practices in Creating DFP Ad Units:
Step 1: Log into your Google Ad Manager account
Step2: Go to Inventory > Ad Units
Step 3: Search for the ad unit in question
Step 4: Click on it to confirm the details like Ad Unit code
Step 5: The Code should match exactly with the div-ID deployed on your page. If you have an ON-page setup, check the ad unit size in the header section of your source code to ensure it matches your DFP ad unit settings.
Step 6: If you see any mismatch, update or change as necessary – either the source code or the Ad Manager ad unit settings.