You may try to lighten the presser foot a little. And a fine thread (50wt) like Aurifil. Sew the patches together by alternating the direction you are sewing them, sew the first two pieces north to south and the next one south to north by flipping the pieces. alternating helps to reduce curving and stretching. Be gentle and slow.
Pressing can cause both these problems too. Be gentle and slow.
another helpful tool, starch.peace