If clipboard redirection doesn't function during a Terminal Server session, Virtual Channels may not be established. Users are able to connect to the Terminal Server, but advanced features, like clipboard redirection and printer redirection, will not be available.
To troubleshoot this issue on the Terminal Server:
1. Press CTRL+SHIFT+ESC to start Task Manager. Verify that the Rdpclip.exe process is running.
2. Open the Terminal Services Configuration snap-in, from Administrative Tools.
3. Right-click the RDP-TCP Connection icon, and then press Properties.
4. Select the Permissions tab.
5. Select the Users icon and press Advanced.
6. Select Users and press View/Edit.
7. In the Permission Entry for RDP-Tcp dialog. verify that the Virtual Channels entry has a check under the Allow column and NOT under the Deny column. Press OK.
8. Press OK until all the dialog boxes are closed.