I have 3 feet that I have been trying out for machine piecing on my new machine. I found that 2 of them work well and one gives me problems, it wants to veer off, and always in the same direction. I used to work in a metal manufacturing shop and so I got to looking at the bottom of the foot and it was poorly manufactured. The foot is not completely flat. I guess one way to check this would be to put some fabric under your foot, lower the presser foot and gently tug on the fabric and see if it pulls out evenly or if it slightly swings to the side. I guess it is like anything else, some good, some not so good. Anyway I thought that I would pass this on as a possiblility to your woes, also.