To me, & looking at your photo, looks like you are not keeping the fabric flat at the seam. Look at the seam behind where you have already stitched it. The seam is not flat & that will cause the rest of the top to travel while sewing & cause the pleat. I hold both sides of the seam taunt & now & then, with needle down, lift the presser foot & jiggle the fabric just a touch then lower the foot & continue. This has always worked for me.