I'm sure you have read the instructions carefully and installed the unit as per the instructions
but just in case there are a few key things you should check.
1) Is the black side of the magnet facing out? Magnets as you know have a north and south pole, if you attach the magnet the wrong way around the unit will not work.
2) Is the distance between the magnet and sensor a maximum of 3mm? The closer you can get this the better. Mine is at about 2mm and reads great. At 4mm it did not work in my case.
3) Are the large flat side of the sensor that faces the magnet and the black face of the magnet parallel to each other when the magnet passes under the sensor? It doesn't matter if the sensor arm goes off to the side as long as these 2 faces are parallel.
4) Does the magnet pass accross the middle of the sensor face? If it only just passes an edge of the sensor it can effect the readings.
The above are the main problems in setting up that I know of. Given that you say you get no readings then the most likely problem is either 1 or 2. If the readings were erratic or appeared innacurate then I would say it would most likely be 3 or 4.