You can just sew a thicker center seam but you will cut off some points. The bias edges have probably stretched during construction. Check to see if they are all still the correct size with a ruler. The only way I have found to correct this problem is to sew in a little thicker seam, maybe one thread width on each point piece. Make sure to stop the 1/4 inch from the center point so you can twirl the selvages points after sewing the center seam.