I don't think the issue will have anything to do with it being run on the SMF platform. It's just that forums are one of the worst converting Ad sites since most visitors are free information seekers rather than buyers.
I've even read tutorial threads on forums as to how to block Ads on the forum in your browser.
To increase CTR you need to get the Ads in the face of the visitor which may not go down well on a forum. The Ads would have to be well targeted to the needs of the visitor too.
My forum signature is under construction.