Here’s a great article from Joe Whyte which lists out the possible filters which might be causing your site to go off the search listing at Google.  He also lists out steps on how to get your site off these “filters” and back in the search results again. Some of the filters include:

• Google Sandbox - Where most new sites get stuck in
• Google Duplicate Content - Pages in your site containing the same information all over.
• Link Farming Filter
• Page Load time - It seems that the bots skip pages which take too long to render !
• .. and lots more.

Here’s the link to the article: Google Filters, how to get around them and exploit their loop holes | Joe Whyte