I have a hp laserjet 2500, it worked fine for las 3 years, but a few week ago start printing only the right half of the page and looks like every time that chance the color cartridge prints less. If I print a page with horizontal lines of colors CYMB each time that toner changes for another line, it start printing more to the right, even if the 5th line is the same color like the first one, it prints more to the right.

I send it to a technician but he didn't find the problem, I have two more printers like this one, and used it to change parts but the problem still. What could be the problem?

Maybe the optical part of the printer on the left is not clean. The service technician could clean the optics as you're saying that it prints on the left but not so dense. If the printer is not printing on the left at all, maybe you have serious trouble with that printer and the service will cost you more. Try with cleaning the optics.