Gran28, this may be a funny question but just have to ask after reading this post a few different times
You aren't moving the quilt after you sandwich it before you pin baste, are you? It's probably just the wording that makes me wonder. And do you use lots of pins? Like at least one every hand width? Are you doing straight line quilting or free motion designs? Is your machine flush with the top of your cabinet? So many variables happen.