I got 3 different "soles" with my Bernina walking foot. The one with the ditch guide caused me all kinds of problems. When I went with the flat open-toed "sole" for my walking foot, I started getting much better results. Also, make sure that when you start your quilting (and pinning), that you start in the middle & work your way out. This is true whether you are doing straight line stitching with a walking foot or whether you do FMQ with a darning foot.
Also, make sure to increase your stitch length when you are doing quilting with a walking foot. Someone else here recommended that to me -- to increase to 2.5-2.75mm and it made a huge difference. My stitches came out more even and the fabric didn't shift so much (I also later lowered the pressure of my presser foot & switched out the sole of my walking foot & that finished resolving the issue).