There are number of reasons why a printer is not being mapped onto the Terminal Server.
Windows 2008, 2003 disables the installation of kernel level printer drivers. The error for this often appears in the server’s Event Viewer. Make sure the blocking policy is disabled. Click here for detailed instructions.
Make sure the client user accounts have permission to install and maintain drivers.
Please make sure you Log off and Log in after adding or removing a printer in the client settings.