I run a site that gets over 30k uniques a day, it is crawled daily by Google. I recently put up a forum on there, the forum itself got indexed pretty quickly but none of my topics seem to be getting indexed, both Google and Yahoo bots crawl the site regulary but don't seem to do anything when they are there?
If you're running vBulletin script, go ahead and install vbSEO and vbSEO Sitemaps hacks. That oughta do the job.
There is a free VB seo hack but apparently has some 'issues', vbseo is definetly the way to go from what I hear if you use VB.
Have you tried linking to some of your posts/threads from frequently visited pages? Maybe a box on the home page "Recent Threads". And make sure your htaccess/robots.txt files are not affecting anything.
You don't need VBSeo or "search engine friendly" urls to get your topics indexed by Google. Do you have good internal linking structure from other pages that are heavily trafficked allowing google to find individual threads? If you just use a standard forum drill down structure and don't internally link to your forums besides forumhome, you will take a long time to get threads indexed. I would recommend the VBSeo sitemap hack though, and that one is free. The VBSeo package does other stuff, but I am not a fan of rewriting forum urls. I consider it a waste of time and effort.
Yep, I have backlinks to topics, they get indexed, but whenever Google goes to the forum it doesnt seem to do anything. Yes, the bots crawl my site and forum regulary, its a very very busy site, but when it gets to the forum as I say it does nought when it is there, it should only need to find one topic then go from there, but it doesnt.