According to the trusty chart that I have, for a finished block of 12 inches (12.5 unfinished) 18.25 is the right size for setting triangles. All I can think of is (1) did you cut the triangles squarely and (2) is the rest of the quilt square. If it's a question of the whole of the quilt being out of square I have no solution but to "fudge" it so that the triangles fit. they may not be 45 degree triangles when you are done, but a man on a galloping horse..........