Do u have the in channel vent visors by chance? That means that it is sensing an obstruction and wont let it close all the way... If there is nothing in the way then maybe the motor is gettin to weak and thinkin there is too much pressure... Take it to the dealer and they can check it... I remember reading there was also an adjustment they could make? Is it running slowly as well?
Probably needs the position sensor reset. It happened to me a few times on my 350z. The last few times it was indicitive of window motor failure. Hopefully that is not the case with the Titan window motors.
Most likely you can find directions on how to reset the window. if not here, then search my350z.com or another 350z board for window reset procedures. It's fairly easy