Well I don't like giving advice since I don't want to be responsible for something bad happening. But the thing is with discord its just so big with so many users that is isn't hard to get people to join, you can make one post of reddit and get 50 people to join in a week so I guess forums just aren't as easy to advertise I guess. Second with discord you can always find a server that is currently already chatting about something, I've been in servers with over 10k people online at a time and there is never a minute where there is not a discussion, but with forums you can post and have to wait hours for a reply not just inducing here, your post might be about something only a few members are going to reply to. I don't think these can really be fixed but I think this is the big appeal of discord. As for advice I don't have much, this forum is very well set out I think it just lacks a way to get new members.