I sympathise with you. It is so frustrating. It sounds like you are doing the right things but I would suggest you pin closer together. I had similar problems when I started and if I was using a polyester batting the fabric slid around much more so I needed to pin closer to keep that from happening and I quilt in a grid like fashion starting in the centre stitch in the ditch and working out then I turn the quilt a quarter and start in the centre and work out and turn again etc. Pinning lots was the key for me. If you use a cotton batting it grips the fabric better and doesn't slide as much. I referred to the Harriet Hargrave book for tips but there are plenty of tips here to help you also. Let us know how you go. I guess there could be a problem with your machine but you would need to try someone else's if you could to determine if it was your techniques or the machine. Good luck. It will be great when you get it sorted.