Josh, limestone is a porous stone that needs care not to use the wrong chemical (no acids) or to much pressure.  For Limestone cleaning it is recommended not to exceed 900 psi because limestone can be damaged at pressures above this pressure.  As far as chemicals, limestone can be cleaned by the SH/Green Wash mix if the stain is organic in nature but since you already tried this approach I have heard of another stain removing technique using a "poultice."    It should be noted that I have never performed this technique but wanted to give you an option to look into. Your local hardware store should sell a poultice cleaning solution for cleaning limestone surfaces.  A poultice is a powdered substance that is mixed with water and smeared over the stain.  The cleaner should come with directions.  I would definately test this out on a small area first.

Good Luck and let me know how it works out!