Getting organic traffic in any forum comes from how high the forum is ranked on Google. If your forum is ranked very down in Google pages, it's going to be difficult to get lots of organic traffic.
With good SEO optimization of your forum, it stands a good chance to rank higher on Google which will in time get you lots of organic traffic. Have you managed to get your forum to rank higher on Google?
I did for one site. Some time ago I had a Harry Potter-flavoured roleplay site called 'Floo Network', at floo.network as a domain. While I wasn't going to win any awards for originality, I did get a surprising amount of traffic for 'floo network roleplay' and similar terms as it turned out there is a really popular Minecraft mod of the same name - and despite how mega-popular they were, I managed to outrank them on Google.
Google hasn't seemed to like any of my sites and it took years before my Christian forum was even in the listings consistently. There were Atheist forums who made it higher in the search results...which made no sense.
I think at one point when I searched "Jcink codes" or the words reversed, my board ranked #1 on Google, but this was for a very short period of time. I don't know exactly what's changed, but I think my ranking on Google has gotten worse over the years, possibly due to a lack of new content being posted.
Personally I'm not so concerned with how high my "forum" ranks (specifically by the domain name, which is the site name) as how high content ranks, which is what I think you mean?
A very simple search term that those in my niche would use to find out how to install an EAF on a particular telescope has ranked near (or at) the top on Google, Bing and DuckDuckGo... install eaf on apertura 72mm.
Same way with a few other of my articles. The AMS content (articles) seems to trend towards ranking higher than posts/threads.
I do have some threads that are on the first page... one is using another very simple search term used by the niche.... startools vs pixinsight.
And surprisingly... even some of my XFRM resources are starting to show up on the top 3 page areas.