A site has PR6 and ranks on second page plus has a lot of backlinks but when I use the cahe:sitename it doesn't show the cache but when I search for the site using its title tag, it appears on the second page and when I click the cache link in the SERP page, it displays cache What's going on?
not possible. If it ranks than it means that it is cached. Are you sure that for example www.site.com ranks but you check the cache of site.com (without www.) or the opposite. It is possible if there is no redirect there.