Welcome to WebmasterWorld Guest from 188.8.131.52
That said, have you thought about using Casale as a secondary agency instead? They are performing well at the moment.
You may be better off using FastClick CPM campaigns as your backup for Adsense. ;-)
Mostly I attribute it to bright blinking spammy looking banners that annoy everyone and make people cringe.
I'm sure if you target the proper blue collar crowd [assuming they even wear collars] you'll get rich:
"Martha, come quick, look what happens when I click on the cockroach - woooo eeee this is fun!"