Like I said before, people grow tired of things not being done.
Small things you might think such as award requests, giving out prizes and koins, but they all add up. People stop asking and move on.
You do the fundamentals right, and people are more willing to contribute, however when it seems as though nothing is being done people adopt the 'why bother' approach.
Things you can do:
What I've stated above
Sort out the moderator positions asap
Competitions to win existing awards and koins.
Advertise ForumKorner on social media more - maybe do a social media exclusive competition.
Maybe also look into doing some external SEO for new markets such as CSGO, Minecraft - a new niche which may spring back some more life into FK.
A concept to actually make groups worthwhile / worth something again.
The majority of threads I see are all to do with oG gamertags and the like. That being said, a lot of people have either just moved on or scammed out, which is probably why there is a lack of activity right now. Having said that, the forum still gets A LOT of activity and anyone who thinks otherwise doesn't have a clue. It takes a lot of time to run and improve a forum such as this, and I hope one day I can also get this kinda activity on my own, but like I said before I believe doing the 'little things' is what really appeases members.
I hope you don't feel I am speaking out of cheek, but I'm sure theres plenty of people who also think this.