How to bring back forum activity?

Hello everyone,
My gaming forum used to average 200 - 400 daily new posts, however; this has gone down to about 50 - 100 new posts a day.

I have tried to promote the forum on the main site, but that doesn't seem to be helping much, even tried to add unique content - but not sure if that has helped.

What would you guys do if this happened to you?

I'm trying to do post exchanges to try to bring back the activity, and I'll see how that goes.