Hi,
After 3 days of struggling with this printer, I finally found this forum, and hopefully you can solve my problem.
I have Windows 8.1 64 bit installed and firstly I tried to print a file in the normal way (not duplexing). It worked flawlessly. But after I went to control panel and after I installed the Duplexing Mode (that was not installed), I went to Adobe Reader and tried to print a 2-page document in duplex mode and it didn't work. I tried opening straight-through output door and it didn't work (I think it is only for manual duplexing... I want automatic duplexing). I closed all the doors, and it didn't work. I installed the driver from HP website, and the printer was renamed to HP LaserJet P2015 Series UPD PCL 6. I enabled duplexing and it didn't work.
What can I do ? Should I open any doors ?