We have been testing a different approach with a client.
It's an inception-stage community. Typically, the community manager continually nudges members to participate. This usually works.
Recently, we're using a different nudge.
If someone isn't participating, we drop them an e-mail. We ask them a few questions about their work, or their challenges, or about their experience/expertise. We ask them for advice on a problem we're struggling with. We write a statement or two we suspect they might agree with.
We don't ask them to participate. We rarely even mention the community. We're finding this achieves better results.
The more participants know and like the founder of the community (or community manager), the more likely they are to participate.
This is especially true in the early stages. You can pester members to participate. Sometimes it might work. Another (perhaps better) approach is to build a genuine, honest, relationship.