HP Laserjet P3005 PCL6 duplex printing

Hi,

     I have HP Laserjet P3005 PCL6,  and I want to print more than 2000 pages, I am unable to do duplex printing, no matter what I do it won't print on both sides of the page. There is clearly an option of "print on both sides of the page" and I selected that as well, in that there is "flip on long edge" and "short edge" i selected both one by one. All in vain, can someone help me please ...