Local printer redirection on remote desktop

Some of my users use remote desktop to connect to machines at remote sites over the internet. When the users connect however only some of their local printers are set up on the remote machine.

In one case there is a hp 3820 connected on LPT1. In the other case there is a samsung ML-1210 connected to another network machine. In both cases the printers are listed as using port LPT1 in the ports tab of the printer properties and in both cases the printer is set as default on the local machine.

I do not seem to be able to make the printers I need available on th remote machine. Does anybody have any ideas as to what may be causing this and what I might try in order to resolve it?