Starching the back and top can help. If it's already sandwiched, use several layers of spray starch. You can use a fan to speed up drying between layers. Starch stabilizes the fabric.
I also wonder if your walking foot is working properly. The purpose of the walking foot is to move the upper layer; it sounds as if this is not happening.
When you took your machine in, did you take your walking foot too so they could check it out? Also, are you using a generic walking foot or a walking foot made especially for your machine. That can make a difference.