Could be a number of reasons,
A high number of your backlinks may have removed your link or changed the anchor link text to your site.
Pr may have gone down on a number of your backlinks,
your competitors may have worked harder on there sites..
I wish I new the answer
If you had these ranking once before, then there is no reason why you can't achive theses ranking again.
If I was you, I would suggest rethinking your seo strategy, look at everything again -even going back to the the basics - continue working on your site as normal, tweaking your site like a bonsi tree - & your show up soon enough!