Take a bunch of measurements on the width and length of your quilt and see how much if any you are out. It could be more of a problem of cupping in the individual blocks rather than the quilt being out of square and if that is what is happening you might need to "tighten" a few seams to reduce the bulk in the offending blocks. Some cupping can be disguised in the quilting process but this can be quite an undertaking. Hope you find a solution, good luck