Thats the same with every forum. I bet you won't find a forum with all members having at least 1 post.
You can have those really loyal members (that post because they want to help) and the ones that come and go all the time, and then the ones who come and leave forever. You can't make people post, but you can entice them. For a discussion forum, the bes would be post exchanges, and a patience.
Also, a good reputation is definitely needed. Why would someone join a forum where the owner is banned, or broken rules on another?
Be careful with the contextual text ads in your forum header. Google won't like the way the colour blends in so closely with the background and it could land you in trouble with them.
My top suggestion for improving your forum would be to get a full TLD domain, preferably .com, .net or .org.
