I've done a lot of quilts that use this strip technique and I have found the error is in pressing ( and inadvertently stretching) the seam, rather than sewing in the same direction. I've experimented different ways and have never found sewing direction to be a factor - only incorrect pressing. So I agree with the others who say to wet it, and re-press it. I have drawn a line on my pressing surface to help me keep the seams perfectly straight, and it does help. Good luck!