Does one page on your site have a far higher Bounce Rate than the others? Check it and see if the links on it are all leading users away from your site. If that's not the culprit, perhaps its content isn't compelling enough.
Provide positive reinforcement when there is no crisis and you'll be able to call on them when there is one. And the corollary is true as well: superstar users, if unappreciated, will leave, and other users will follow them out of your forum.
Because sticking a bunch of people into a windowless room and telling them to be creative is going to be about as effective as sticking a gun to their heads and commanding that they write a guaranteed hit song.