What are the keywords you've lost rank for?
There are a lot of reasons for losing rank:
- your competition is doing more SEO than you are
- you have added a lot of content, but not optimized the content for those particular keywords, so they are getting diluted
- Google no longer perceives the page/site to be as valuable as it did originally (check who is now ranking above you in the search results)
- you have flooded the site with your keywords (spamming)
- you are linking to bad sites; bad sites are linking to you
- poor content and/or duplicate content (especially after the Panda updates in Google)
This article is very helpful:
http://www.rainbodesign.com/seo-tips/go ... g-drop.php.