abacus, could it be the pressing that is deforming the shape??If you are sure you are sewing exactly or pretty darn close, to a 1/4" seam and all looked pretty good till you ironed it...could be the way you are pressing your fabric. I know, for I had to fight this for a long time...I like to press hard and go from left to right...a big no no in causing the fabric to move. There are many free videos out there to watch w/the proper way to iron. I have to hummm the words in my head still:))In fact, I just might not be very safe w/an iron:))LOL Just a thought on that subject...Skeat
*If you don't have a 1/4 guide on your foot...then use quilting tape to mark your line on the machine itself to give you a guide.