Remember the 4 fundamental things a community provides its members?
These were a sense of belonging, mutual support, greater influence, and exploration.
You have to satisfy at least one of these.
You need to plan this. Don’t hope it happens.
If you want to increase the sense of belonging, you encourage/facilitate the geekiest/hardcore discussions on the topic. You enable off-topic discussion. You use a high level of self-disclosure related discussions. You cultivate and highlight these sorts of interactions. You engage straddlers in these interactions.
If you want to increase the influence of the community, then you need to identify the things they want to achieve. You need to work with your community to create a plan to achieve this. You act as a community organizer. You champions the milestones, rally the community on the disappointments. You document the progress and the contributions individuals have made within the community. You promote the community activities and achievements through traditional media channels.
You need to pick at least one, and push hard to make it happen. Otherwise, there is an excellent chance it wont.