I had the same problem at first too when I used pins to baste my quilt sandwich. Looking back now, I think I pulled the backing fabric TOO tight when clamping it on the table and that caused stretching and uneven areas that then caused tucks when quilting. I switched to spray basting instead of pins and have far fewer puckers in my backing fabric. So if you want to stay with the pinning method, be sure you have your backing smooth but don't pull it too tight. Seems counter-intuitive but try it.