If you're trying to convert newcomers to regulars, you need to align the entire conversion process around a single motivation.
It's tempting to treat the process as a series of humps. You can identify a different tactic to overcome each hump
For example, you release exclusive information to get people to visit the community. You offer a free ebook to members that register. You launch a challenge/competition to get members to make their first contribution. You use a game-mechanics system to keep members active (perhaps even offer tangible rewards for the winners).
But this approach doesn't work. The reason why a member crosses a hump matters as much as crossing it. 10,000 members registering to get a free ebook is less valuable than 100 registering to exchange tips with experts in the field.
You need prospective members in the right mindset when they cross each hump. Sure, there are a number of proven methods to increase conversion, but none of these affect the fundamental motivation.
Now for the scary part. If you've tried everything, and it's still not working. You need to make a big change, not a small tweak.
You have to do two things.
First, you need to make a fundamental change in your community concept. This means changing who your community is targeting (narrow the focus), the type of community you're developing, the positioning of the community, or the benefit of the community. This isn't easy. You need to do a lot of research to get this right.
Second, you need to get really good at articulating the benefit that captures this motivation.