I recently realised one line of friction with a forum is that we discuss in threads aka topics, which have a subject line, and need to be created.
One can see with email - which is decreasingly used - that some people struggle with subject lines as a concept, or as a challenge. Whereas chat apps just give you an audience. Even with slack or discord, where chat happens within channels, a normal user doesn’t start a channel, they just pick one.
So, I think starting conversations is something you need to do as a founder, or your active members need to do. Perhaps start a conversation around a topic, or perhaps just semi-regular - a weekly discussion thread.
You need to host and facilitate the group: ask open questions, prompt people for their thoughts, spark something off.
Try not to make head posts which are declarative, unchallengeable, or just too long.
Try for head posts which are more like “here’s an idea, what do you think?” or “here’s what I’ve been thinking about, how about you?”
Make sure there’s a fresh head post at least weekly.
But for sure, the pace of posting and responses will be slower on a forum than in a chat channel. The advantage can be that you’ll get deeper thoughts, more reflective answers.