ok, this is what i would do to tackle the problem.
first off don't press the seams open - 90% of the time seams are pressed to one side in quilting.
we typically press the seams to the dark fabric.
this way when you join opposite color rows the seams will nest with each other allowing for near perfect matching of seam lines - sometimes you have to ease the seams and thats why i say near perfect.
the next thing i would do is length your stitch. the pics aren't wonderful but it doesn't look like your bobbin thread is being pulled to the top and causing the puckering.
i think the shorter stitch length is whats causing the puckering.