Decorative stitches can be problematic for anyone. I think the problems (for me at least) occur when the stitches build up a little bit too thick and then the fabric doesn't feed evenly. The feed dogs are pulling the fabric through, and your hands should only be guiding it gently. You might want to experiment with changing the stitch length and/or stitch width on your machine until you are satisfied that the decorative stitch will work with your quilt sandwich. Practice on a quilt sandwich using the same batting and similar fabrics until you have it adjusted to your satisfaction. Some of the advice you've been given is excellent advice, but meant for FMQ (free motion quilting), not for your current situation. The speed you're slowing at doesn't affect the length of the stitches when you're using the walking foot (feed dogs engaged) and you can't do decorative stitches when you're doing FMQ.