Great suggestions so far. How did you sandwich it? If you used pins, you may need more. Starting in the center is a good way to go, if there's a small amount of fullness, it will be pushed out to the edge. For straight seams a walking foot works well. Finally, it's not a good idea to practice on a real quilt. Take the stitches out, make a couple of small practice sandwiches and get used to the process, then quilt your real quilt.