How did you put the border on? If you have a strip of border and start sewing or pinning from one end to the next, you will get waves and not it will not lay down. Measure the middle of your quilt. Cut your border to that measurement. Divide the quilt in four sections with pins. Divide your border piece in four sections and match the pins up with your quilt and border. Then ease the material in between with pins. Sew the "baggier' part on the bottom and the feed dogs will help ease that material. Doing the border this way actually squares up your quilt. If you do it the other way, the way that was mentioned first, you may actually add length to your quilt and that is where the wave comes from, and it is a nightmare for long arm quilters to quilt.