Getting a HP duplexer to work after you've installed it.

Note that on HP printers, even after you have manually installed a duplexer into the rear of a model such as those for the HP 600 series, it will not necessarily work unless you make changes to the printer's properties. Select the printer under Printers & Scanners in Windows 10, click Manage, and then click 'Printer Properties'. On the Device Settings tab, scroll down and find the option in the 'Installable Options' group for 'Duplex Unit (for 2-sided Printing)' and make sure that it's set to 'Installed'.