You use poly batting but you don't say which thickness. If you are using the 1" thickness you will get problems. Try a thinner more like 1/4" thickness. This one will still more slightly but with pins 3-4 inches apart should be stable. Also you should stabilise the quilt from the centre outwards before doing free motion or lines. Lines should be done opposite directions, I do centre to edge along edge to start new line to other side edge then along edge and back to centre. ( roughly a figure 8 from centre). My friend did all her rows the same direction and made a terrible mess of movement to all be undone.