When was the last time you had an expert review your meta tags for relevancy and how competitive they are?
Have your key search phrases changes recently with new search behavior and/or changes in your industry? If you have not checked in the past 90 days, you are overdue.
Are you getting plenty of quality content added to your site on a regular basis?
How about internal and external links? Internal links help the search engines index your site, identify primary keywords and can increase the page rank of linked pages. External links or links leading to pages outside your website do a couple of things. Like internal links, external links help the search engines identify important keywords. Let me know if you need help establishing more internal and external links.