Cleaning a dark patio advice please

searchlight123

i have a very dark grey / black limestone patio that badly needs a clean. i have a karcher pressure washer but having done a third of the area, now its dried, it doesn't really look much different to the uncleaned area!! very frustrating. i just used the water, but it's hardly made a difference so am looking for advice on what i can use to help bring this patio back to it's former glory, (it was laid around 4 years ago and sealed at that time also). my feeling is it needs some sort of chemical to help shift the dirt but i do have a dog and need to be mindful of that also.

many thanks for any tips and recommendations you can offer.