I have only recently been trying my hand at free motion and I don't have a for sure answer for you but in talking with other quilters, my guess is that it could be the speed? If it isn't constant along with the movement I think thats when this type of thing happens.... but I would wait for others with more experience to answer!! LOL Sorry I can't be of more help but I am sure someone will come along that can help you better! Good luck!