It's just a time thing. Google, or any other spider for that matter, very, very rarely crawls and indexes an entire site in one swoop (unless you have a mass of high
PR and trusted backlinks). Especially if it's a new domain.
An
SEO mod only increases rank in a small way by the weighting it gives to the url, it doesn't increase spider activity.
Robots.txt won't really help, a site map might, but it's a forum, so as Google notices activity and growth it'll visit more frequently and indexing will increase in size and frequency as well.
A few high
PR backlinks will help activity as it immediately establishes some kind of trust.