I have never had a bar that hasn't slipped regardless of the brand of walking foot or machine. It is not just a Janome problem. I don't use them because they are unreliable and it is difficult to turn a quilt with them on because once I flip them up so I can turn the quilt I can't be sure they are in the right place. If I'm doing straight line quilting I try to do it so it lines up with the corner of squares or some other reference point on the quilt and then I don't have mark as much nor do I have to deal with the bar.