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Also experiment on the color. If CTR is low, it means your color may not be working enough.
The key to Adsense is experiment, experiment and more experiment.
1) You have a lot of regular visitors, and they're often seeing ads they've already seen.
2) More important, your visitors aren't researching ways to spend their money, so there's no reason for them to click on the ads.
Some sites work better with AdSense than others do. I do very well with a travel-planning site, but that's because people who are planning vacations are actively looking for travel services. Similarly, a site that reviews cameras, vacuum cleaners, networking gear, etc. may have a good clickthrough rate because users are researching purchases. On the other hand, a news site like Washingtonpost.com probably has a fairly low clickthrough rate because people who are reading about the war in Iraq, a Redskins football game, or tomorrow's weather aren't likely to notice or click on ads.
Have you tried banner-ad networks like Fastclick or Burst Media? It's possible that you'd make more money with CPM ads (ads that are sold for $X per 1,000 impressions) than with contextual text ads like AdSense that require clicks to earn revenue.
2- Do you have a default setup? I think that you are probably getting an unusual amount of PSAs.
3- It could be the subject of the blog, as alluded earlier some topics are just not good for making money in advertising or changing topics might give adbot trouble with its targeting.
Worst case scenario - most of your users are repeat visitors, the ads aren't of interest to them, the same ads show on most of your pages, users have seem them over and over, your pages are long (spanning the fold) and by the time a user is done reading page content the ads are no longer on screen and they're unlikely to remember them and scroll up, review them and click.
If you're not paranoid, add your URL to your profile and/or stickymail it to me and/or others who'd be glad to review it and advise or provide a lot more detail.