Clean the glass and suction cups before installation. Use isopropyl alcohol or a quality glass cleaner. It is also helpful to install the suction couplers in temperatures above 70 degrees (this helps soften the cups up a bit and creates a stronger bond to the glass).
Connect the suction couplers to the support bar before installing in vehicle (do not tighten the thumbscrews). Work from one end first and press the cups firmly against the glass. Be sure that both cups are fully engaged against the glass. Extend the support bar until the opposite suction coupler presses against the opposite window. Be sure that both cups are fully engaged against the glass. With all suction cups fully engaged, extend the support bar as far out as possible (you do want to put some side pressure against the windows) and twist to lock. Tighten the red thumbscrews at the ball & socket connection.