There should be more interesting and really engaging topics and maybe each member should promote the forum or share their content to social media platforms. In that way, more and more would be joining.
I think that if there are more interesting topics there would be more users and traffic on ones website. Users will be more engaged with new forums or threads and updated responses.
You have provided good medium to keep engaging users. however, i would also agree with @Jason76 that letting them have a signature and talk on politics will help fetch lots of posts from users.
This is what I do to keep members active and I try to tell my long time members to do it as well but often times it just falls on me to continue the practice and that's difficult when I'm trying to also do administration responsibilities. I run out of time to do it all.
Asking questions to something someone wrote is one of the best way to get people to interact.
If someone says "I like dogs" you don't just say "I like dogs too" because the conversation ends there. Ask that member "Which breed is your favorite?" That will keep it going!
Creating new threads with value also encourages members to post. A lot of my long time members want to wait for others to create interesting threads and that means slow periods as they wait. Why won't THEY start threads?
You have to give something to people in return. Something that is value to them. Otherwise nothing takes off. Even Facebook and other social media are used as tool to vent off.
Following up with user's content with questions help with keeping the conversation going. Replying without an open-ended post is not enticing the other participants to reply to you, ultimately closing some conversations down.
It's hard, even if just your staff members or yourself posting it looks darn bad. even when you see good amount of traffic coming in you not always get new signup or any new content. Content gets so old so fast. People just read but not always reply