I've heard that Google and other search engines have a "crawl budget" which means they have a finite amount of time/effort they will spend on a particular website to index it. It could be that this "crawl budget" is exhausted before your forum is completely indexed, and the newer threads are prioritized.
If this is the case (and Google doesn't exactly make clear what's happening, so we have to guess), you could try to exclude non-important pages from index in hopes that the crawl budget will be used in places you consider more important. For example, maybe Google doesn't need to index member profiles or other low-content pages when it could be indexing informative articles or high-impact discussion threads that get you traffic. I hope that helps.
Wow.
Google gives priority to Reddit, very often with garbage/promotional content and ignores forums like mine with millions of posts over a 20 year period which contains gold information.