Not all ad units are displayed
AdSense detects the number of ad units that are on your page, and automatically serves unique ads to each ad unit. The system is optimised for pages that have highly targeted ads available. As a result, you may sometimes see that only the first ad unit contains ads. As our ad inventory changes or if your page content changes, you may start seeing more ads on your page.
When only one ad unit is displaying ads, any additional ad units will display as transparent boxes, or will display the back-up ad or colour that is defined in their ad code. Please note that each instance of ad code must have a URL or colour specified in order to display back-up content.