As far as i know... Google dont allow the acct holder to ENCOURAGE OTHER to CLICK THE ADS. It should be click by their own will.
Forum visitors are generally repeat visitors and they turn ad blind, as they become aware of the spots in your site which are useless to him, so keep shuffling your ads here and there and change the color of the ads too, you might see a change, may not be a major one neverthless you will see some spike in clicks
I have a forum, only guess can see Adsense Ads, I disabled it for registered members as they don't click on those Ads.