So is your forum Search Engine Optimized? AdminBB is, as xenforo is Optimized out of the box. What about your forums? Are they optimized, have any special mods installed?
The Old Pluto Hosting Support Forums previously used VBSEO and then later phpBB SEO for SEO Friendly URLs and that's about it I think.
I know SMF, IPB and vBulletin all have search engine friendly urls now, but you should also edited it to include header tags and such.
My MyBB forums I've ran as of late all have Google SEO (in priority of what's really needed). - XML Sitemaps and 301 redirecting duplicate content - rel="canonical" - robots.txt - h1/h2/p tags in multiple pages (this was manually done in MyBB) - Friendly URLs - Dynamic Metas (though description is now the only important/useful one) IPB/xenForo have most of it except for the Sitemap.
Gotta agree, its very simple and all but its does a good job. And yes for mybb Google SEO is the best.
I run on IPB and it is optimized out of the box. They also released an official IPS add-on called IP. SEO which I use. The first few months were dreaded for me. I didn't know if my forum would stay alive or not but after some time it finally started to pay off. What was once 1 new registration per day turned into 20 in the beginning of March.
Took time to get my forum search engine friendly, but nowdays it is Several mods, and tons of edits in phpBB3 code.
Thats what I hate about phpbb, you have to do a ton of work, and template edits to install a simple mod.
I would say my IPB forum is optimized enough. Most of the SEO work on site for forums are the forum links and some custom pages. Other than that, I think off site SEO becomes increasingly more important for forums as you want to gain members.
How much organic traffic do you get? I know vBulletin 4 is pretty good for search engine friendly URLs, and if you build backlinks with it, then you'll do fine on search. Search engine friendly urls = good for organic results, and if someone is searching, they'll come across your site if you have a lot of backlinks/
Last time I checked, it was about 25%. Not a very high number but this is pretty much without any modification done except for adding on IP. SEO which is still lacking when you compare it to the vB alternative, vBSEO. Oh and "enable FURLs", that's all I did.
Actually Google SEO is great, thats probably all you will need. And sitemaps help also, if you do ever get it, be sure to submit it to Google Webmaster tools for faster indexing.