When I find that one section of the top is approx 1 1/2 inches shorter than the piece it is supposed to be joined with! Could not figure out how to stretch it so the pieces did not pucker. Ripped it out the next day, steamed it and left it to sit!
In the middle of the next night, I awoke from dreaming about strip quilts (yeah, yeah, I know!) to come up with a "Fix" - at least in my mind! We'll see!
Best wishes for a solution for your outer border!