Since the center was flat when you started, and supposedly square(?), then I agree that the problem is probably with the borders, especially since you started to see wrinkling when you sewed them on. Rather than taking off the stabilizer, I would take off the borders and see if the center is then flat. If not, the problem is with the stabilizer, but if it is, then you need to square up the center and apply new borders, cutting to size first.