What i found when quilting with fleece backing, and I have very little experience with doing that on a DSM, is that Stitching on the straight grain - across or down - is generally ok after you have a bit of practice. Stitching fleece on the bias is like all bias stitching, more stretch, more problems. I suspect that the problem is that the fleece on the back of the sandwich is moving with the batting and top. Maybe using a walking foot would help?
One other observation on fleece: WOF on fleece seems to have the least stretch, too, where as on 100% cotton, the least stretch is on straight grain (length of fabric).