If this a sewing machine, unless you have the stitch regulator, you can't match the same speed at which the sewing machine can with doing FMQ so your bobbin thread will pop up. That is why I was taught back in the 90s to use the same thread on top and the bobbin. All feet used doing FMQing has to pop up to allow you to move the quilt. A walking foot will stay down since it moves the quilt as it stitches. Hope this makes sense.