Originally Posted by Kobi's Q
After having mine for 6 months I think the sensitivity varies quite a bit between how much light is available and how that light shines on the windshield. I have found that sometimes the thing is overly sensistive and will wipe with barely anything on it and then there have been times when I thought it wasn't turned on because I could hardly see out with the amount of rain on the windshield and it had not wiped once yet. I have noticed if I manually do a wipe by moving the lever all the way down and then move it back to the rain sensor position it takes over very nicely.
I find I have to give a single manual wipe when first getting in the car.
I put it down to there being a film of water to start rather than drops for it to count and decide to wipe. Once there is a clear screen drops are easily counted from them on.