I’ve had the issue twice. Both involved high humidity when I canvassed. I think cedar expands a lot in high humidity. Now I never stretch unless the humidity is under 50%. I have a dehumidifier in the shop. Rather than start over, I pulled the staples and stretched it again. It was easy on the sides because I never trim the canvas until after paint, but the ends were tricky. BUT it did work out fine... You might try that as opposed to starting over.