I lengthen my stitch to 3 and 3.5+ if the batting is extra thick when using a walking foot.
As said in previous answers, make sure your quilt sandwich is laying nicely so it feeds through evenly. You may have to start and stop a lot to adjust it, but the results will be much better
If the decorative stitch you are using is intricate, it can give you problems. I would suggest the Serpentine stitch, or one that is very open and simple. Change this stitch too, longer and maybe a little wider also, but check that it is not too wide for your foot
As you get more experience, you can try out more complex stitches