USB sticks not working on thin clients (Sorry if it's in the wrong area)

I have several thin clients, wyse, hp and would like to introduce more but have one major problem. I need for people to be able to use usb memory sticks and also usb cd drives. Currently they are all linked to a terminal server on our network and every thing's running fine.

I just can't get any of the clients to detect any usb sticks (or other devices for that matter such as card readers). There is an option in their settings called 'port security' that has an option to allow usb devices but even with this activated nothing works. Do I have to make any changes to the terminal server end? hence why I placed the thread here.

Please could anyone shed some light on how I may resolve this as I can't find anything online.

Many thanks

James Brownridge