1) Check the server error of web site, is its 200ok or not, if not than some computer program ( particularly google not index the web site) if website in not index and cache then you not obtaining ranking by adding distinctive content, meta tags, EL tags etc.
2) Check web site home page is cache or index ( if home page not index than overall traffic and ranking is down)
3) Check if canonical issue is gift or not
4) Check any duplicated content is gift or not
5) Check if any mirror website is already existence in web.