Used to have one of those, they dont really cool much =p (according to me)
They do cool, but it basically only works with convection. so: it cools the AIR by creating friction. It's physically impossible to cool thermal radiation (warmtestraling) with this, that's why sometimes it seems like it doesn't work.
Well I'm not saying that it didn't work at all, more that it did cool but too little for me to actually find it useful. But yea, that might have had to do with what you said aswell, I honestly wouldn't know =p