Since updating from Reader 9 (which usually let me print in duplex) to Adobe Reader X, I can no longer print in duplex.
I have verified that my settings are correct for the printer, two-sided mode is highlighted in the printer dialog. However some searching has suggested that you now have to go to to Properties » Advanced » Duplex Settings and choose Duplex for it to work.
The problem with doing that is that on the document I am using the duplex settings option is grayed out. Its important to note that the document isn't locked - I can highlight just fine.
Is this the case? Who would move printing options away from the print driver and into the app? I've burnt about 200 pages because I'm trying to print out a developer manual for some on site work, and I still have another 300 pages to print.