Check that your tension is not too tight. I Found out the hard way that too tight tension causes tiny ruffling of the border, and that stretches the fabric out. I had to remove a border that had been sewn on with too tight tension. When I measured the removed border off the quilt, it was 2 inches longer than the quilt.
You could say that I learned my lesson...and you'd be right.
Also, when adding borders on a quilt, I start sewing them from the center to one end, then reverse and sew the rest - also starting at the center - of the border to the end. It has really helped me with wavy borders.