I have a few of these sensors and they are more critical then the X10 model. If you look close at the body of the main sensor there is an arrow at the bottom just to the left of the words Radio Shack. The magnet has an arrow at one end of it. If the arrows don't lineup right on... the magnet won't trigger the sensor or it will be hit and miss. Double check that and let us know.
(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)----KDR
Another thing I just thought of... make sure your cover is on the unit when you set it up. There is a tamper switch built into the unit that will trigger the sensor when the cover is removed.
(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)----KDR