Another thing that can cause skipped stitches is flexing of the needle. If you move the quilt sandwich too quickly, the needle is caught up in the sandwich and doesn't come down in the correct position to form a stitch. Sometimes a bigger/stronger needle solves the problem, and sometimes moving more slowly is required. I don't do sitdown quilting and don't have your machine, but I suspect adjusting the presser foot pressure can also affect the flexing of the needle. If the quilt sandwich is more compressed, the needle is less likely to be deflected by it. Everything has to work together in a sewing machine.