u/anchickens

The headstone is located on the plains of Colorado, so we don't get a ton of moss or lichen on our headstones here. I tried to clean it last October using Wet & Forget and a natural fiber scrub brush. I didn't notice any difference 3 months later. Was it too cold for it to work? Please let me know what I can do to make it look better. It used to be bright and pretty.