Ok, on one of my blogs I have two google ads in the side column. It is a 120x600, block of text/links below it, then another 120x600 adsense block. I was just proof reading one page, and oddly enough, the first 120x600 at the top of the page was displaying a public service announcement, while the second position was showing normal ads. It was my understanding that PSA's are shown when there are no relevant ads available. Obviously there are relevant ads when they are being shown in the second block. Even in the event that there were only enough to fill one block, shouldn't the sponsored links get the top ATF placement and the PSA get the inferior placement?? I don't understand this. I've encountered it a few times in the past. The funny thing is that if I click refresh, the PSA usually goes away (not always, but usually).
It isn't only about relevance, it is also about pageviews. If you are the only one refreshing it, you will notice that it will change to PSAs Once visitors start hitting that page, it will begin serving ads.
I've seen this happen too. Does not really make any sense, but I've only seen it a couple of times so I have not really worried about it.
Weird things definately happen in low traffic situations. Maybe the OP can tell us the number of uniques per day to the page in question (I suspect it is either really low or the site/page is new)
The site I am talking about in this particular event is low traffic because it is a new blog. However, I have also seen it on another site I have with 50,000 uniques per month. That's not the highest traffic out there, but 1,500 unique per day should be enough. Anyway, I'm not sure that it has to do with the volume of traffic. Maybe, but I don't think so. Even if it does, I don't understand why PSA would be above the fold and links below. It still seems like it should be opposite of that.
Well, if you want to test your theory about their not being enough ads, you can try Shawns AdSense Sandbox http://www.digitalpoint.com/tools/adsense-sandbox/
It's not really my theory - just that's the situation I had the impression would generate a PSA based on something I read long ago. Google says that these are the possible reasons why, but I don't think any of them apply to me. Our system has not yet crawled all the pages of your website. Your page may contain sensitive content for which relevant paying ads will not be displayed. Your account may be disapproved or awaiting review. Your site has restricted access using a robots.txt exclusion. Your website is using session IDs in the URL. Your website is using frames. Your ad unit is set to display image ads only. Your web page may not contain enough content. The AdSense ad code was placed within an IFRAME. Your web pages are behind a login. There are too many URLs added to your account filter list. Your site content is primarily in an unsupported language. Your ad code has been modified Your page contains a refresh tag.