There's a few possible explanations for this issue, the most common being:
The ad code has not been inserted correctly.
The best way to check this is to go to your site, press ctrl+U to bring up the source code, and do a search for the word BidVertiser. Then compare this code to the one in your Get Ad Code window. Obviously if you cannot find the ad code at all, then this is why the ads aren't showing.
Low traffic websites (and sometimes new sites).
Websites and blogs with a very low number of page views are being removed automatically from our ad server after having a very low stream of traffic for a certain period of time. Once we get a request for an ad from such site, we put it back in the system as an active site, a process that may take a few seconds. If you will refresh the page that serves the ad a few times - you will eventually see the ad running. Once your site will have a constant stream of traffic - the ads will show up constantly with no disturbance