I have finished weaving five dish towels on a straight twill warp using 8/2 unmercerised cotton. Each towel treadled differently. They all came out lovely and soft, and four of them are nice and rectangular. But the one woven in straight twill came out warped, noticeably not to square It's a perfectly serviceable dish towel, but it's a weaving sample, and I want to learn from it so I don't do it again. What what would cause this?