I done all work SEO for my all sites according search engine guide but PR has been down. is google's monopoly? help
Depending especially on the age of the site, sometimes PR can vary during Google's dance. Also some have found that modifications to the home page during the optimization process can have short term unpredictable results.
Seo newbies often have a misunderstanding of PR building... you really need to learn more to be good at it.