You'll need to check the Pages without Revenue report in Big Data Analytics and then check some of those URLs to determine why they did not bring in revenue.
If they are showing ads, it's possible the issue got resolved or perhaps the URL just didn't get sufficient pageviews. Even if a URL does get pageviews, ads may not show depending on the type of traffic (e.g. bot traffic).
You'll want to make sure there are no issues with objectionable content and that you haven't intentionally disabled ads on those pages.
You can use the Ezoic Chrome extension to verify that ad placeholders are present on those pages.
© 2021 Ezoic Inc.