Hey Scott,
You get the lines because you're cleaning that area twice (the overlap). Also, you could be etching the cement if it's not able to withstand the pressure you're using. When I get them I can normally get rid of them by spraying the surface with Sodium Hypochlorite (let it sit for 5-10 mins and they normally disappear). You don't need a high concentration, maybe dilute your 12% 3:1 or 4:1. Should be strong enough . . . probably stronger than needed. If that doesn't work then I've had to go over it again sideways. Try the SH first.