Wondering if it might have something to do with the set-up for free motion on your machine, such as lowering feed dogs, setting the presser foot at the right height, etc. Does your manual tell you how to set for free motion? I'm no expert but it seems like each machine needs it's own settings. If you've used this thread for piecing without any problem, I would check into other causes. By the way, I use and like Aurifil for piecing and free motion but have also used others just prefer that one.