I'm not an expert, but I do belong to a list of Sapphire owners so I've read a few things.
You could try a stretch needle, and maybe experiment with the pivot height in the tool box. I don't know which way to set it, though. Just remember to take notes of the original settings before you change them.
Which foot are you using? I think the spring free motion foot would be better, because the spring holds down the quilt sandwich more.
Janet