when you stop trying to use your zipper foot . . .
and get yourself a darning foot.
Oops. I guess that's what happens when you lose your sewing manual and don't have any real life quilting friends. There's no one there to say, "You're trying to free motion quilt with THAT? You do know that one's for zippers, don't you?" Um, nope. I had forgotten, due to extreme zipper anxiety. I finally got over it when I agreed to sew a couple of sleep sacks for a friend by copying one she already had. I started to work out a way to put velcro on them and realized that it would be about 5 times more complicated that just taking a deep breath and sewing in a zipper.
For those of you who are non-sewers, the thing is that you need to be able to move the quilt around under the needle to free motion quilt. And a zipper foot, as it turns out, is made to hold the fabric in place so you can sew straight along a zipper.