Have you looked at the transfer roller to make sure it is level in the toner cavity. If not then it maybe stuck down or even the spring maybe missing. If you have checked for this then try a half test. put a sheet of paper in the tray 1 and run a test page once the paper is in the printer open the toner door and look on the page. you are looking for where the lightness starts. also look on the drum of the toner to make sure the print is dark there also.
The mirrors in the laser/scanner unit can collect with dust and cause this, having rules easier solutions out. There is a small polgon mirror that spins and tends to collect dust on one edge similar to a ceiling fan. The laser unit is under the top cover and has a black plastic cover with some cables and a few screws. See here for more assistance. http://www.feedroller.com/Store/hp_manuals.php