(Taken from MS Knowledgebase article #302361, http://support.microsoft.com/kb/302361)

This problem occurs because the printer port does not begin with COM, LPT, or USB. Ports such as DOT4, etc. will not be redirected.

Force all ports on the client computer to be filtered for redirection. To perform this, create a DWORD value named FilterQueueType under HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Default\AddIns\RDPDR and set the value to FFFFFFFF.