One of the main reasons that there may only be public service ads on your site is because the Adsense "spiders" can't find any content on your site that is appealing to advertisers.
One of the best solutions would be for you to re-work the content on your site and / or at least add some high-paying keyword content to your site.
If you would like to have a copy of a
"High-Paying Keyword List"
just visit my The Adsense Consultant Blog - Part II where you will be able to order a copy of the high-paying keyword list.