same thing happened to me. on mine it was the motor cutout switch located in the tailgate latch on the left side of the tailgate. its a switch that breaks the circuit to the motor so you can't roll it up when the tailgate is open. over time, a part of the switch will get bent and not function correctly. long story short, remove the latch from the tailgate and inspect the metal shaft on the cutoff switch. you can bend it back straight, or just buy a new switch from lmc or something.