Several possibilities here. You could have your soap too strong and it could be drying before your rinse. You might just not be getting all the soap off. Worst case scenario is one like I had with one of my accounts. The water you are using is too hard. Doesn't matter how hard you work at it or rinse at it, with black trucks it's very hard to keep streaks off with hard water. Only solution to this is to use de-ionized water. Are you using a brush or two stepping?