You may have seen Google AdSense showing PSA or public service ads on your site instead of actual ads by which you get paid. Problem with PSA ads is that you don't get paid for the clicks on them. This article is my experience with PSA ads. 

This is how AdSense ads work
For sites that add AdSense it takes some time for AdSense crawlers crawl all the pages. The ads shown are determined by the page content. If a page showing ad is not on Google AdSense indexed then chances are very high they will show a PSA ad and you won't get paid for clicks on them. If the page is not indexed then they mark it for crawling. Note that Google AdSense crawlers are different then google.com crawlers.

What can you do?
Generally it takes about 1-2 days to get a site crawled. Just wait until then, you should be receiving actual ads, not PSA ads. Just remember Google is loosing money too showing all the PSA ads, so they will make best efforts showing actual ads as soon as possible.

My Experience

When I started AdSense on webcosmo.com first two days I saw a huge display of PSA ads. I realized this won't make me enough to pay certain expenses I intend to cover by the ad money. I was seriously thinking of getting rid of AdSense and use some other services. Then I decided to give it some time, without knowing its gonna go away. Things became allright after 2 days. I am reaching my goal with AdSense now. Problem with webcosmo.com was there are more then 50,000 pages. A huge number of pages are added everyday. I am sure it took some time for AdSense crawlers crawl all the pages. If you have less number of pages, you probably gonna have PSA ads for less duration compared to me.