If it is a mitered border corner, gently remove the stitching trying not to stretch the bias. Iron the corner and iron in the 1/4 inch fold on one bias edge. Lay that folded corner over the other side. Does it lay nice and flat now? If so put a few tiny drops of Elmer's washable glue on that 1/4 inch folded seam allowance, lay it on top of the other side and press until dry. Take it to the machine and sew on the fold.