We have a drive towards self-validation and construct a large portion of our identities from social signals we receive from others.
A large part of community activity is driven by that self-validation drive. If you run a community where people share knowledge, they're probably doing it less to help and more to get these positive signals from others.
Or, in layman's terms, we spend a lot of time taking actions designed to solicit positive opinions of ourselves from others. We gather these opinions through a range of implicit and explicit signals.
Offline, we notice signals by what is said, how it's said, the body language of people, how people react to us, and hundreds (perhaps thousands) of other relatively minor details. For example, if someone abruptly ends conversation with you to talk to someone else, that's a signal. If there is an empty chair next to you, that's a signal. If most people in a group are facing you, that's a signal. If someone returns your call quickly (or not at all), that's also a signal.
Online, the signals are either explicit or non-existent. You either receive positive responses from your contributions to a community, or you don't. All those tiny, crucial, signals we pick up offline are absent online.
Some tackle this by gamification. But gamification is an incredibly blunt instrument for something as complex as this. We care far less about points than gamification advocates imagine. This is also true of recognition. Recognizing members is a tenet of community managers, but too often recognition is used in its most primitive form i.e. "tell a great member and they're great!!"
So what signals do we need to work on and make more explicit?
There is no 'one big thing', but hundreds of tiny things. For example, the speed of response to someone's posts, the number of responses, the content and language used in those responses, the calibre of people that respond to contributions, the broader impact that contribution has in the community.
There are all going to be major signals that members to subconsciously satisfy their drive for self-validation. If you can make these signals a little more explicit, a little more obvious, it should increase the level of activity in the community.