There are several possible reasons that the wrong ads may be rendering on your website. Some reasons are related to ad trafficking, while others are related to the code configuration on the page.
Wrong ad unit
Each ad slot needs to be associated with the correct ad unit. Verify that the ad unit exactly matches the Ad Unit Code in GAM. If there is a typo, or the target ad unit is archived, you may still receive programmatic ads. In reporting, these will be attributed to a generic ad unit named “Default”.
Wrong ad unit code
The data-unit attribute must match the Ad Unit Code in GAM - NOT the Ad Unit Name.
Wrong or missing targeting
If targeting is missing or incorrect, an unexpected ad may render. Check targeting using HTL Debug or the Google Publisher Console tool.
Wrong geo
If the expected ad campaign is restricted to a specific geographical area, and you do not live in that area, then you may need to use a VPN.
Zipcode Targeting
GAM has limited support for geo-targeting on cellular devices. See the Google documentation for additional details.
Missing creatives
If a line item does not have creatives for every ad slot size that appears on the page, or if the same size appears multiple times, but creatives have not been duplicated, then one or more ad slots may serve the wrong campaign.
Incorrect creatives
This can happen! If everything else looks correct, verify that each creative image or script are correct.
Conflicting line items
If there are multiple eligible line items for a piece of inventory, GAM will choose among them according to the Line Item Selection algorithm. The selection occurs in two phases. First, GAM determines line item eligibility, according to targeting parameters. Second, GAM determines line item priority, based on the line item configuration. If two line items are both eligible, and both have the same priority, either may be served.