Many times puckering is the result of too much downward pressure being exerted by the presser foot on the quilt sandwich. The fabric ends being pushed back toward you as you sew, resulting in tucks and sniggles when it meets an obstacle.
I found that this UTube video explained it so I could understand it.
http://www.youtube.com/watch?v=Ox8qR...eature=related
Hope this helps.