In addition to square, lso verify that you are level. I found I needed a big 3-4 foot long level to find the dip in the floor at my friend's house. In that case it was the front center support that was low, the outer corners were just fine. My friend is tall and the adjustable feet were already out a bit, but the right piece of cardboard did the trick.
So does the machine drift on it's own anywhere -- like if you put it in the middle with no fabric on the frame does it move at all? My guess is it would go the same way as your issue.