Two Types Of Participation Problems
If you have low levels of participation, you need to identify if members are visiting but not participating, or if few people are visiting.
The difference should be obvious. If people aren't visiting, then it doesn't particularly matter what you do in the community, no-one will see it. You need to focus on reaching these members. You need to develop clear tactics to promote the community.
That usually means building a permission list of members you can interact with. For example, interview 10 people about their top tips in the topic, publish that as an article members can download (you get the e-mail addresses), then invite those people to the community (the people you interviewed need to promote this).
Or you might interview popular bloggers/influencers and ask them to post a link on their sites. You can also directly invite people you see elsewhere to join the community. Make sure you are inviting them to do something specific in your community (and not just join a community).
If members are visiting but not participating, you have a relevancy problem. The focus becomes seeing what they do when they visit the community. Do they bounce at the homepage? If so, make sure you highlight emotive discussions on that page which members can participate in (and make sure this is above the fold!).
Do they browse/see the discussions but not participate? Change the discussions they see. Use sticky threads to highlight the most engaging/controversial posts. Research your audience. Identify the topics they really care about. Start discussions based upon their hopes and fears.