Try this, because a common mistake many people make with tent canvas is to try and pull it too tight. First, when you "pop up" do you attempt to run the cables as tight as they'll go ? If so, back them off a little to give the canvas more "slack" to reach he extent it has to to cover your openings. Many pop-ups have stabilizer rods that you install after cranking up to set the weight of the roof on and takes the tension off the cranking / cables system. Secondly, if your canvas "roof" poles have adjustments on them, back them off one hole or two again, to create more slack in canvas.
Pulling canvas as tight as possible to have a taut, tight trailer everytime you camp WILL wear out your canvas alot sooner than expected especially trying to cover around outside corners, etc.
Just a thought or two but have seen this problem in past.
Good luck and enjoy !!!!!!!