If the printer has a duplex unit installed but does not print automatically on both sides of the page, check the driver/software settings on the computer to make sure that the duplex settings are configured correctly.
These settings tell the printer how to print the page and override any settings on the printer. The duplex option must be added in the driver and then turned on to enable it to function.
This is usually the case regardless of the data stream (i.e., PostScript, PCL) or operating system. Please follow the instructions below.
Note: The following descriptions apply specifically to the LexmarkT630 driver for Windows, but the instructions are similar for any of the laser printers.