/forums ranking for main keyword, main site not

I operate an 8-year old site that until this summer was getting 10k-15k worldwide visitors a month despite not being ranked in the top 100 for the main keyword (say "XYZ"). For many secondary keywords and phrases, the site ranked very high due to excellent content. Since summer, visitor count has dropped to 7k probably due to one of the Google updates.

I never did any SEO on this site until recently where I paid someone to help boost the ranking. Now when I check the site for the main keyword XYZ, the site forums rank between 100th-200th place on google, but the site is not ranked at all. (Note that the site still ranks high for secondary keywords and phrases, usually about related forum pages. So Google has archived the site, and it has a pagerank of 4).

Why would this be? I rarely update the site these days (have 100 pages or so of content in it and can't add much more truly useful stuff on the subject), but the forums are updated daily via new member posts. Maybe that is hurting my site ranking but helping the /forums page ranking?

Also, on each page of the site, I have two links at the top -- Home (to the home page of the site), and XYZ Forums to the forum page. Perhaps Google is thinking that the forums represent the XYZ subject better than the main site because of this?

Bing and Yahoo both rank my site much higher than the forums for the main keyword.