iPod Touches and USB Hubs

So, I have four iPod Touches in my class and I would like to connect them all up to charge them via a USB hub. I can sync them all on the hub, but I can't charge them all. I get a message saying charging is not supported with this accessory. Sometimes one will charge, sometimes two, but sometimes none. Has anyone else had this kind of problem and, more importantly, found a solution? I am using a Macbook OS X 10.6.5, a Targus 4 port USB Hub, and 4th generation iPod Touches. Thanks.