Measuring the proper distance between your towers is critical for proper, tight installation. When tightening the Q Tower to the bar, make sure to keep the towers on a level surface so one doesn't rotate slightly.
A properly installed Q Tower will have a slight bow in the crossbar. This is normal, and should be considered good reason to give yourself a pat on the back. Make sure the pads on the car don't move around on the roof of the vehicle by giving them a couple of taps from each side.