Default ads serve when no other ads can serve. Here are some reasons this can happen.
Your standard ads are not scheduled to serve. Check the ad start and end dates for your placement. Make sure that these dates are correct. If the ad start date is in the future, the ad cannot serve yet. If the ad end date has already past and you set a hard cut-off, the ad cannot serve anymore.
All your other ads use targeting, and the user doesn't meet the targeting criteria. Check to see if you’re doing any country, device, or audience list targeting on non-default ads. If you want a rich media creative to serve when your other ads can't, make sure it has no targeting criteria and that the only ad that has a lower priority is the default ad. (The higher the number you set in your ad properties, the lower the priority.)
All non-default ads use frequency capping. If a user has disabled cookies or device IDs, frequency capped ads will not be eligible to serve.
<noscript>section of the tags will serve and the default ad will appear.
Use non-targeted ads
To prevent a default ad from serving, make sure there are other ads that are eligible to serve. For each placement, we recommend adding at least one ad that doesn’t use any targeting. If users don't fit the targeting parameters of your other ads, Campaign Manager 360 can still serve the untargeted ad. However, if you don't have any untargeted ads available for that placement, the only option is to use the default ad. If your ads are narrowly targeted, users may see a default or untargeted ad more often.
Another advantage to non-targeted ads is that default ads don’t allow rich media creatives, but a non-targeted ad does. Adding a non-targeted ad ensures that these kinds of creatives can serve to users who don’t match your targeting parameters.