You are cleaning from the top down! While that appears to be the logical thing to do, the soapy dirt that flows down over the still-dirty wall below tends to create streaks. Always clean exterior walls from the bottom up. Work on the shaded side of the house and move fast enough to keep the whole area you are working on completely wet until you have reached the eaves at the top. Dirty water flowing over the clean surface below won't have streaks, and keeping the wall wet until it is all clean and rinsed keeps any dirt from drying on the surface before reaching the ground.