It's going to be a tension issue. What I would do is get two pieces of scrap fabric and a piece of batting. These don't have to be large pieces. Just big enough so that you can see if you have adjusted your tension correctly. Adjust your tension so that the stitches are even on both sides. Without knowing what kind of machine you have and how old the machine is, this is the best way to tell you how to check to see if your tension is right. Before I quilt the tops I always have a scrap "sandwich" (meaning one piece of batting between fabric) to test the tension. Always check this because the different fabrics, threads, and batting can change the tension. One other thing that I have learned the hard way is if you free motion quilt, go real slow around curves. If you don't, your stitches will pull in the back and it looks awful. If you need anything, just ask.