I own a forum and have a strange situation within google although google do send me traffic (about 3,000 uniques per month) I seem to rank well for a LOT of things never seem to get the really high quality/quality traffic, I have nealry 40,000 pages indexed and most rank top ten for there search terms but my top page is only PR3. Does this make sense? my adsense earnings are fairly low as well.
Most sites rank well for a lot of things, but if people aren;t searching for the things you rank well for it's not going to result in much traffic.
Don't use Google ads on your forum. You won't earn much..integrate CPM ads on your forum..you will earn more from them. I Totally agree with stOx as well.
Most good forums with a lot of members are updated every minute usually with new posts or topics. Just imagine how many posts are made on this forum within an hour. I'm surprised the Google bot ever leaves.
I like using adbrite on forums and then set your own price for the ads. Or just sell banner spots here in the BST. Good Luck.