Two things...
First, are your practice pieces the same weight as your quilt? Are you practicing with the same batting and thread? If not, it could still be a tension issue.
Otherwise, try slowing down! If I move the quilt (my hands) too quickly through a curve I get nests on the back. So I chant it like a mantra: "Slow. Slow." Seems to help me a lot.
Good luck!
PS - C&C might indeed be the problem. However, my machine loves C&C and HATES Aurifil. Experiment!