I had the same experience with a quilt I finished last Fall. This tip helped a little: pull corner-corner and it might help a little.
A tip I learned at Guild to prevent that; measure each border as you put them on. When adding borders, be sure to start in the middle; not at the end. They all said this is true, but no one had a good reason for it. This helps keep it "true." Constant measuring and trimming if it isn't exact is how to overcome it.
When I had that experience, I actually took the quilt apart and started all over. It was stupid of me, but I felt everything just HAD to be perfect!(big mistake) Enjoy!
Mariah.