I see it is happening as you try to sew over a already quilted verticle seam. Going over a quilting lines is always a tricky spot. I make a frame with my hands around the foot about a 6 inch circle with my Machinger gloves on. I kind of pull the quilt outwards with my hands as I sew and watch to make it smooths out any extra fabric. If you can't smooth out the surface before you get to the line, you are going to get a pleat or pucker. I try to choose other quilting designs that do not have bisecting lines for this very reason.