Just think of how you lined the triangles up the first time, you used the whole side of the triangle. But now you have folded seam allowances, so to line up properly, you'll want to open the folded seam allowances temporarily so you can line up the triangles from corner to corner. Once the seam allowances go back in place, you may have little dog ears, but that is fine. I hope that makes sense. Just imagine each triangle without the seams folded back and line up the whole sides.
Rachel