Sometimes people don't want visit a platform to interact with each other.
At least not at first. Not all online communities will be developed on a traditional platform.
…and that's ok.
We have a bad obsession with the two-step, platform-led, approach. Step 1) Get people to visit the platform. Step 2) Get people to interact.
The problem with this model is that step 1 is just one of several possible approaches to achieve step 2. If you can get people to interact in other ways, then you don't need a traditional platform (yet).
In fact, it's usually better to build social capital before you launch the platform.
Just because this is the most visible model, doesn't make it the only model (or even the most common model). There are other ways to get people to interact with each other that don't rely upon a discussion board.
This is especially true for communities of practice, hyperlocal communities, and communities in which people can meet in person.
Hosting a weekly conference call and recording the notes can help. Organizing a regular meet-up works well too. In many cases, it's easier for someone to subscribe to attend a regular event as opposed to a never-ending series of discussions.
How about having people send one tip a month on a challenge they're facing, or give one recommendation a month. That's a simple trade-off to create a wealth of knowledge and builds social capital between members.
If your platform-led approach is struggling, try something else. There are no shortage of approaches to develop a community.