I am doing seo for one site, PageRank is 4.. last one week site is not cached.. if i checked cache:mydomain.com, showing not found.. what is the reasons..
Google like fresh content. If you update fresh content in your site. your site will be possible to get re-indexed.
Google keeps changing the crawling time depending upon the new content you add. If you have not added any new content in last few days then this may happen. The best way to see if your site is visited by google is awstats.