Try some mechanics hand soap... specifically the one that has pumice in it. Most hardware stores sell it in a gallon pump jug. The best one to get has natural orange in it. It smells like fresh oranges and its gritty and creamy. All you do is poor it on thick and scrub, and then rinse off afterwards with a garden hose. It's also environmentally safe.
I swear it will take the dirt or grease off damn near anything. Just to give you an idea of it's power...I had a white shower stall floor with stains that would not come out with any household product that my wife tried...including bleach, etc...So as a last ditch effort, I tried the mechanics soap with a small brush and scrubbed. It worked, the shower stall looked like brand new when done.
I think it has 1000 possible uses as a cleaner...good luck.