Question : Problem: printer Duplex Unit

I have an HP laserjet 8000N print device, I connect to it through TCP/IP port.
in the device settings tab, I chose Installed for the Duplex Unit.
But when I print it doesn't print in both sides.

any idea ?

Answer : Problem: printer Duplex Unit

And another thing, I don't think the 8000N includes a duplex part, you'd need require the 8000DN for that.