Are the other strips in the design supposed to be equal width? In the photo the grays don't match each other, nor do the horizontal yellow ones. If this is not how they're supposed to me it might be part of your problem as the triangles will never match. If that is the case you need to double check your fabric pieces before that are sewn and make sure you have an accurate 1/4" seam allowance. If the widths are correct though, matching seams by pinning straight down at matching seams and then adding two more pins at the sides usually works.