I have done as you suggested, Margo, and I think I see what's causing most of the problem. When I tried the tool combination on a smaller test sandwich with plain fabric on the back and front and the same batting inside, there was no problem. But then my quilts are never just solid fabric, back and front, so that didn't do me much good.
Then I tried the same thing on a very small piece, 14x14 or so. The top is a leftover trial of the watercolor quilt idea - remember those? Lots of 1 inch squares which had already been quilted in the ditch. With this piece, the stitching did not hang, producing the teeny stitches which I kept getting on my larger piece. BUT. The fabric bunches up toward that spike and its little cap on the circular attachment as the quilt makes its circuit around it. My attempts to keep this from happening causes the circle quilting to become distorted.
In the photo (which I hope will show up here, I have attached it), the two circles with the blue arrows show you what happened. This shows the back of the little quilt. Both those flat sides were on the final arc of the circle. The circle with the green check mark in it was the third attempt, and turned out better, but only with great effort at smoothing out that humping toward the center. The circle with the red check mark was done with the walking foot alone, along a marked line, with no circular attachment. The red one is about as good as the green one, with a whole lot less stress.