Some machines have the feed dogs spaced further apart than others and this makes it hard to get the "perfect" 1/4" seam. I find I need to adjust the needle placement to the right or left so that the fabric is over both feed dogs.. I then re-measure and mark the location. I have the Janome 7700 and I use the accufeed for most of my pressing and I have found that the presser foot that is wide enough that I can simply align the needle so the outside of the presser foot is my scant 1/4". I use a ruled 3x5 note card to measure my 1/4" -- the lines are 1/4" apart.