First, you need to figure out if what you are looking for on your site isn't indexed yet, or if it just doesn't rank well yet.
To find out which it is, you should do a Google search in the format "keyword site:yourdomain.com". If you don't find the thread you are looking for that way, then google hasn't indexed it yet. If you find the thread you are looking for that way, then its indexed but other sites rank better than you with that keyword.
A lot of factors decide where your site ranks, one of the most important factors is inbound links.
I definitely can find my post if I search with (keyword and my site domain).
But when I search for any thread title, keyword, or even entire sentences, I can't find my posts at all.
I have used the tool from Attracta to submit site map to Google.
I can even see Google spider on my forum under "Who is Online" most of the time.
It is not simple ranking low, it seems to be no where to be found.
For example, I made this post "walking a bird" on my forum.
http://petskeepersguide.com/forums/Thread-Walking-a-bird
Then I went to post about my post on this forum with a backlink to my own forum.
http://www.city-data.com/forum/pets/1518067-walking-bird.html
Now if you Google keyword "Walking a bird", the second link shows up on the first page of search results. Mine is no where to be found.
I obviously created my own thread first, with more content too. So what did that forum do which I haven't done yet? In order to have the new thread/posts to show up that quickly on the search engine front page?
I also wrote two articles so far. One is "fishless cycle"
http://petskeepersguide.com/fishless-cycle-nitrogen-cycle
Now if you Google keyword "fishless cycle" or "nitrogen cycle", my article is no where to be found...no where to be found.
All the views were done by readers who came from the direct link I posted on Yahoo Answers as a part of reply to answer their questions.