If you were working on a large 3 or 4 rail floor frame the backing was stretched to tight and the top wasn't stretched tight enough. I worked with a Grace 3 rail system several times before getting my mid arm quilter and I had this happen. Every time I loaded the silly thing I had to stop and think which needed to be tighter the top or bottom. I never basted when I finished the quilt while on this frame, but I did pin baste with this frame so I could machine quilt. Basting for machine quilting is much easier on a frame than on the floor
....... Good luck!