Stitching with a walking foot, a generous quarter inch on either side of the zig zag line formed by the Flying Geese units, which calls for continuously swinging the quilt from side to side.
Flying Geese Quilt - backing
Eventually I worked out the the sewing machine's timing was thrown out by super-bulky seams, and if I ran the walking foot alongside the seam line rather than on it, the skipped stitches disappeared. Then by adjusting the needle position the quarter inch distance from the seam line remained consistent. Phew!
Still so much to learn.