They probably have a Towa Bobbin tensioner. This sets the tension on a bobbin, but when it's off the machine. If different parts of the quilt are pieced with different fabrics or lots more piecing in one area than another, then this can throw the tension off. If there's no fabric differences, then is the tension off on just curves? This might mean that they are going too fast on them. With some stitch regulators, you still can't go from zero to 60. May need to slow down on some of the curves.
I agree with Peckish about taking the quilt back to the long armer and asking.