Does not boot off mountain lion usb bootable drive

So I have created a mountain lion USB boot drive because I work on a number of imacs, but for the latest generation it doesn't seem to work.

I create it in the normal way, downloading the app through itunes then using http://liondiskmaker.com/ to create the usb boot drive.

When I try to boot from it: http://i.imgur.com/IUDlv.jpg

I get this icon on the screen: http://i.imgur.com/DNc8U.jpg

The weird thing is that I have used this exact bootable usb stick to upgrade many, many older imacs to mountain lion. I don't want to have to redownload the entire 5gb or so every time I need to install mountain lion on a new imac. I've tried this with both the 21.5" and the 27" varieties and both have the same issue. What is causing this?

I also need to boot from it directly rather then doing an in-OS upgrade in the odd times I'm working with an imac that is not bootable or has a bad hard-drive.