Knowledge communities commonly use the Motivation, Opportunity, Ability (MOA) model for their community efforts.
If you have a community that isn't sharing the quantity and quality of knowledge you need, it's a problem with one or more of these three elements. Each can be dissected.
The most common problem is motivation. Members simply don't feel like sharing information. Motivations are intrinsic (people doing it because they enjoy the process of sharing knowledge) or extrinsic (people doing it for an external reward; praise, recognition, money etc...)
It's easier to use prizes/gamification, but this does more harm in the long run. Members tire of rewards and participation plummets.
Members are more likely to share knowledge if:
a) the community is active, no-one wants to waste their time on an inactive community.
b) their contribution will have a significant impact. Self-efficacy is a powerful motivator. People will share knowledge if it will positively impact many people.
c) it helps integrate them into a group. If sharing a lot of knowledge is seen as a standard way to feel part of a group, more members are likely to do it.
To increase the motivations of someone to share knowledge in a community, you can tweak these. First, remove any dead/quiet areas of the community. This makes the community feel more active (see social density). If you have no discussions, initiate a few and then directly solicit people to participate in those discussions to get things started.
Second, highlight the impact which contributions have made. Have a place that shares details of the best advice shared, how it helped people in the community. Keep a rundown of the best advice of the week. Create a wiki/eBook of the best advice shared in the community and it's impact.
Third, encourage a sense of community. A key part of this is off-topic discussions. Members are far more likely to share knowledge with people that they like. However, if they can only talk about knowledge sharing issues, they will never get to know people. They might only visit the community when they need information. You need to allow and encourage other discussions/activities that help facilitate these group bonds.
You don't have to go wildly off-topic here, but you do need to encourage members to get to know each other personally. At least, have discussions that let members blow off steam about the biggest challenges/frustrations they have.
Members need the opportunity to participate. Of course, for online communities members always have the opportunity to participate. We need more than that. BJFogg would call these triggers (make sure you watch the video on his site).
How would a member know when to participate? What triggers this behavour?
If visiting the community isn't a habit, the answer will be never.
For professional communities, most people visit during work hours. Pick a time, perhaps during lunch or towards the end of the day, and aim to trigger a visit then. This can be via a daily outreach e-mail. That e-mail could included unanswered questions, latest success stories, or tips you feel all members should know.
Alternatively, you can have a lunchtime quicktips feature. A twitter-length tip based upon what members are working on during that particular day. Now members know that after they have lunch they browse the community.
Having a list of unsolvable questions that is updated during a lunch hour can be effective too. You need to trigger the behaviour. You might also let members contact specific individuals for advice - if they are particular fans of that person. Tagging members they want to reply in posts can provide simple, direct, triggers.
Ability here comprises of two elements.
The first is the literal ability to contribute. Do members have the time, money, and resources (computer with internet) to participate? You need to have a simple homepage in your community that reveals all the relevant questions and new knowledge in a single location. Your members need to be able to scan this in a matter of seconds.
The second is the ability to contribute good knowledge. Health communities would be terrible if the advice didn't come from doctors. If you the majority of your members are new to the topic, it will be difficult to share high quality information. The problem for many communities is to attract, keep, and motivate the experts to participate.
That often means a deliberate recruitment effort.