I have a manual awning and what you describe occasionally happens. (I'm assuming your roller tube is not bent.) Cause: The outboard edge of the awning fabric has slipped out of position in the attachment groove on the roller tube. The fix: Extend the awning all the way and lower the front as far down as possible. Eyeball the fabric to see which way the front of the fabric needs to move to square it back up with the top of the fabric where it attaches along the top side of the trailer. Stand forward or aft of the roller tube (depends on which direction you need to move the fabric), brace the end of the tube with your chest so that it will not move when you tug on the fabric. Now, grab the fabric in a pinch grip with both hands, right at the roller, and pull/tug. All you are doing is realigning the awning fabric so that the top and bottom edges are square with each other. Roll the awning back up slowly, observing if further adjustment is needed. Seldom do you need to move the fabric more than an inch. Repeat the process until the awning rolls up and locks in place properly. The last step is to again extend and lower the awning and, with a marker pen, mark the roller tube to show where the fabric should be when properly adjusted.
It is probably the same problem if you have an electric awning. But you may have to be on a ladder to move the fabric. Ouch!
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.