I use poly batting all the time without problems shifting.....the key is basting well (glue lines about 3-4" apart)....I am more inclined to think that you need to reduce the pressure on the presser foot and to make sure you have the right foot for the right job....walking foot to do straight line quilting with the feed dogs up.....hopping foot/darning foot to do fmq with feed dogs either up or down (personal preference) with stitch length set to 0....on my machine, if I want to do fmq, I also have to set my thread tension tighter (8-9) to get a good stitch.