HP Laserjet 5550 A3 duplex Problem

[alunmjones]

im having problems with getting the printer to properly print duplex, its a manual duplex, it prints on the first side fine, but when i turn the sheet over and place it in tray 1 to get it to do the second side it comes out with a curved creased line in the paper, i fixed it for a little bit yesterday but its back and its worse.

 

i cant see anything wrong with the inks or any of the rolers and ive taken it appart as much as i can during a free lesson.

 

any ideas?