That shouldn't be happening. The bunk ends should sit flush with the trailer box. My first thoughts that the canvas is getting caught on the bunk slides. With the top up, push them in until they stop, then go around and see why it's stopping. If the canvas is hanging and you can unzip the panels partway, that may make it easier. It could also be a cushion or something sticking up. That will generally catch on the leading edge of the bed. Also, check the bed rails towards the inside ends and make sure they're not bent or damaged.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition