I also posted this in the General Discussion forum, but it seemed better suited here:
Have you considered breaking down your forum topics in greater detail? Are you up for a little 'brainstorming session' on this thought?
I have found that being very specific with the topics encourages more thought-provoking discussions... for example, we have an internet marketing forum here, but separating/adding the SEO (search engine optimization) forum has proven successful in stimulating that specific topic of discussion