If you sewed on the biding with a walking foot, it should be at the right tension. There might have been a little wave in the borders that can show up after binding. I have found that the Quick Machine binding with flange seems to help in controlling waves. The extra seams in the binding make for a firmer edge. You might try the machine binding in a wall hanging and see if you like it for a flatter look.
For now, I might use a needle with invisible thread and run a little length of almost a gathering stitch where the waves are. If you put a good knot on each end it will pull the edge in enough to take the wave in but not enough to look like its gathered if you do it from the back.