AdSense detects the number of ad units that are on your page, and automatically serves unique ads to each ad unit. The system is optimized 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 backup ad or color that is defined in their ad code. Please note that each instance of ad code must have a URL or color specified in order to display backup content.