I was just talking to one of our team members about this, since some SEO practitioners in our market had clients that ended up wiped off of Google in last December's update. He helped to go in a do a complete audit of backlinks in order to help repair some of the damage done by the other guys.
You should do a thorough sweep of the backlinks and see if any of them were torpedoed by the latest Google update. If so, that might had hobbled your traffic and reputation somewhat. I'd also check your analytics of top referring sites now vs. a time of peak traffic, that might be able to lead you in the right directions as well.
If I can help you more with this, just give me a call.