You should try to ensure every post in your community gets a response within 24 hours.
Members that don’t get a response to their posts, especially their first posts, don’t return. In addition, single-post threads is the community equivalent of boarded up houses on the street, it shows a lack of engagement in the community. Why would a member post on a forum when so many aren’t getting replies? It’s a downward spiral.
Set two times a day when you respond to posts without any responses. You don’t have to have all the information, you can ask for others to help e.g. reply with “great point, Mr. X, I’d love to know what Harry, Joe and Melissa think of this” or “I love this topic. We had a big debate about this a few months ago, check out the thread here: www.link.com”.
You can also enlist volunteers here. Have people responsible for topics reaching out to those whom post in that area. It doesn’t matter how you do it, just make sure that every post gets a quick reply.