One possible problem is uneven quilting, another is longitudinal quilting. when I was new to the art, I thought it would be a great idea to finish my border with several longitudinal rows of quilting. The result was always wavy borders. I see some people who don't seem to quilt borders at all, or do it very little. Even if your measurements are right, these can cause wavy borders. I had one very wavy border to which I added a bunch more quilting. Problem solved.