When you pin, do you make sure your backing is slightly stretched before you add the batting and top? If not, you can get puckers. You said you use 'too many pins'. They should be 3-4 inches apart. Also, when you SID, do you start from the center? You should start at the center and sew out to each of the 4 sides to get it stablized, then continue to work from the center as you complete your quilting.