It could be the wrong type of Teflon sheet. Is it bumpy on both sides? I have bought different types of Teflon sheets for my oven over the years, and I cannot remember any of them being bumpy on either side. Also, flannel will probably never slide as easily as regular cotton.
What you might want to consider doing is elevating the quilt so that you have only a small area of the quilt in contact with the surface at any one time. Jenoop is a commercial setup that is available, but there are other ways to elevate. Someone recently posted how they attached long clamps to their sewing table instead of using PVC.
Here's a link to the thread about using long clamps, plus a link to the Jenoop:
Stop the Quilt Drag while FMQ
http://www.jennoop.com/suspenders.html
https://www.youtube.com/watch?v=gJGph8am6BE
and one more
https://www.youtube.com/watch?v=mfOZpLy-Dpg