If your 1/4" foot has a little light metal edge, it can get bent or sprung out. All it takes is getting a pin jammed in there to push it out enough so that it is off. Even when these feet are perfectly straight, they can still be enough off to make your seam allowance inaccurate. Sometimes you can adjust your needle position just enough to compensate for your foot. I'd try bending the edge in a little, change your needle position and if you're still unhappy, try another foot. There may even be a generic one that will work better for you. May you soon have happy stitching and accurate too!