The rain sensor works by the principle of refraction (bending light???). Try sticking your finger into a bucket of water. See how your finger appears to bend? That's refraction, at least the way I understand it.

From your description, it sounds like the problem can be fixed by simply cleaning the outside of the glass in the area of the sensor.

If the wiper leaves even a light film, or streak of water in this area, the sensor will request another wipe. You can check this easily, by looking closely at the area of the windshield where the sensor is located, during a wipe. if you see any streaks, or if a slight film of moisture remains, clean this area thoroughly. I've everything from regular car wax to glass compound, with good success. Of course, make sure the wiper blade isn't the problem. Let us know how what happens.