But some of the posts with even seo friendly title and threads are not showing, does that mean they are not indexed by google ?
You can tell if example.com/some-folder/some-page.php is indexed at Google by searching for [site:example.com/some-folder/some-page.php] without the brackets. My guess is that the URLs are more than likely indexed, but they are simply not ranking for the keyword phrases for which you are searching.
If your forums are setup with good navigational structures, the engines should be able to easily discover and index your URLs (especially the first X pages/posts within each forum). But the deeper and deeper forum posts/threads fall into the archives, the less likely it is that they will continue to be indexed and to rank. Once a thread is 25 pages deep in the archive for example, it's not being pasted nearly as much link juice as if it were linked to on the first couple of pages worth of post/threads in that forum. So over time, if threads don't acquire external links, their rankings often dwindle and they can even get deindexed at some point if you have tons of activity on the site.