If you did them already, how about sharing a picture? That way, it would be easier to (maybe) see where you went wrong (if you did), and offer some helpful suggestions.
Thanks.
And, in the meantime, here's a Sharon Schamber video on mitering corners. She uses a block as an example for ease in showing the techniques, but it could apply to a king size quilt.
And even if you don't use every single one of her pointers, at least you'll know what you should be looking for, and how/what to fix it when you find it.
http://www.youtube.com/watch?v=zAAa66kJRr0