i have a forum. but some time i get public service ad sometime get ad how ? in this forum i use two adsense account. my forum www.famesocial.com
That means Google is not founding the perfect advertising matching to your content so just add new content and update it ...
Doollar is most likely right. Keep adding new content to your site. Also, try and stick to one general topic per page, so Google can provide the right ad match.
I assume you just place adsense code for not a long time ago. Most of public ads displaying on publisher websites comes from their new status. Googlebots do not acknowledge your website content theme yet, and most likely do not know which type of ads would be displayed. Please be patient and wait for a week at least. You would see the public ads changed.