Google Crawling and Indexing

Hello,

I have been running a forum for about 6 years.
Some months ago i upgraded the board into Vbulletin software.
I have almost 1 milion page views per month and about 90.000 posts.

What is happening is that i only have about 2 or 3 pages indexed by google.
The index page is updated almost daily, but then nothing else is being crawled.
I have another page indexed at google, with cache date of may 2005.

My board is open to guest's and i rarely see googlebots online crawling. Other like yahoo are always crawling my board.

I'm using now sitemaps, updated daily and i have link exchange program to help increasing crawling and rank, but nothing!

How can i solve this problem?
Thank you.