There are a couple of reasons I can think of that this might happen.
First, make sure that the printer KNOWS that you have a duplexer installed. I think this will show up under Paper Trays and Options if you print a configuration page from the printer.
Second, if there's an option in the printer control panel to tell it that a duplexer is installed, make sure it is on.
Third, is the printer connected directly to a machine, or "shared" from a print server? If it's shared, then tell the controlling machine (the print server) that the duplexer is installed under the printer properties, Device Settings, Installable options (it should be right there, or somewhere thereabouts.. .I have a 4000 I'm looking at the settings on; it may differ slightly for the 4050).
If you're using a JetDirect card, make sure that the firmware is up to date.
If you're using XP/2000, make sure that you are making the changes to the device settings while logged on as the administrator.
Hope all this helps!
Just my $.02
"In order to start solving a problem, one must first identify it's owner." --Me
--Greg