I see different reasons for your problem.
While your entry page has a PR of 5, most of your other content has a PR of 2 or 1. Your main page has content that is found (exactly formed) on many other sites with much higher PR, so you get buried in search engine results.
The subjects you are trying to get found in on search results are very competitive. You need to read up on techniques for distributing PR properly. You've got links and links and links going around and out of the site. It needs to be more focused if you want to keep some of that front page PR. You may even want to consider banning links inside the forum or at least force text links instead of hyperlinks so again you don't bleed PR.
I am not certain it would help at all but you might want to explore making your links static looking instead of dynamic (not ? but / in the path). Google only likes dynamic links up to a point.
Last but not least did you do a domain name change recently or is it that are you cloaking your content on another domain (which I won't mention). That will get you in trouble fast if it's on purpose.