^ I was wondering that too.
I have to say that my even if my window is clean they do not improve. I am pretty obsessive about keeping the car clean (ignore the state of my car in pictures in my reversing sensor post...).
I tried the on - off - on trick the other day without any improvement.
I also don't think I can see any difference when I slide the 'speed' setting from low to high or vice versa.
May be a duff sensor..
Current: 2013 F31 320D M Sport Black, 403M 19", ProNav, Black Leather
Previous: 2013 F30 320D M Sport AW, 403M 19", Bi-Xenon, Black
2005 E91 320D M sport, BSM, Twist 230 19's, Lemon
2002 E46 320i M Sport, Convertible, Titanuim Silver, 19
1998 Lotus Elise 111S, Black