There are a number of reasons why a video does not appear on your site.Check the Ad Unit HTML code is correctly embedded onto your site by enabling the "Test Ad Units" function on the upper, right-hand corner of the Dashboard and Ad Unit sections.
Correctly embedded Ad Units will produce a demo ad when "Test Ad Units" is enabled. If no video appears, try refreshing the page and confirm the compatibility with your site's platform (Wordpress, WIX, etc) or Ad Manager (DFP). Until your site is approved, no revenue earning advertisements will appear on your site. You can enable the test mode to preview Ad Unit placement prior to site approval.
Please note that Ad Units embedded in the <head> of your site may not produce a demo ad.
Typical reasons why your Ad Unit is not earning revenue, or no video appears on your site may be that the visitors to your site are not within the campaign target audience for reasons such as their browsing device, location, interest profile, etc.