I recently launched a new site and I noticed that the AdSense ads were not all showing. I have noticed in the past that adsense ads, when first launched, showed white blank spaces. But after a minute the ad would appear. This new site, was the first where I put the maximum allowed, three, ads on a single page. I knew the issue was not an issue with Norton Software Blocking AdSense because I tested this on several operating systems and browsers. So I started a thread over at Search Engine Watch forums named Multiple Ads - Different Sizes - Should it Work?.
I at first thought the issue was that Google might not allow different sized ads on the same page. But that didn't make sense, so I asked in the thread. Soon after I received some excellent responses. David Wallace quoted the AdSense Support page:
Can I display more than one set of Google ads on a web page?
Currently, AdSense publishers may place up to three ad units on one web page. Our system will detect the multiple ad units and will display unique ads to each ad unit. This system is optimized for pages with highly targeted Google ads - some pages may not show ads in the second or third ad unit. In this case, the ad unit will show as a transparent box, or will contain any alternate ad or color specified in the ad code.
But Jenstar, WMW forum moderator gave us a gem.
When you ad multiple ad units to a page, I have noticed that it will usually only show ads in the first ad unit - sometimes for days - before they begin appearing in the second and third. Not sure if it is due to an inventory issue, a low priority for newly added multiple ad units, or perhaps they respider the page to get different keyword combos for each ad unit, but I have noticed this many times. It is rare that I see ads in the second and third ad units when viewing a newly uploaded page for the first time - and those ad units without ads will seem to vanish (to prevent three PSA ad units appearing on the same page). So it is a consistent problem, but one that often makes publishers go back and recheck their code to try and figure out what the issue is.
The problem should correct itself within a few days. On rare occassions, there could be a consistent ad inventory problem with not enough ads available, and those second and third units will remain blank. But a quick check with the AdSense Preview Tool can give you a quick idea if there is inventory and whether those ads will eventually show up.