I think the softness comes from the enlarger lens, probably not closed enough or enlarger misaligment. I have seen this before, the center very sharp and the corners soft. The fact that you dont have light fall off indicates to me that it might not be the taking lens. Although Don has a good point, you might have had enough coverage but ran out of circle of "good" definition.
Have you checked the negative with a 10x loupe and see if the corners are soft in the original?