It seems that this pucker is a matter of not enough pinning or basting, as was indicated by other posters. I have just been through the same thing. When I ripped the one seam that had the pucker, laid the quilt flat and repositioned the pins (very close together), and restitched letting the walking foot do it's thing, it worked out. The next quilt, I needle-and-thread basted close together and got no puckers.