My Gpa bought a Wii and had the same problem. Then sensor bar is exactly what it is named. An Ir emitting bar(infrared). Many things produce this like Candles, and the sun.. [0000----------0000] (fun project look at your sensor bar with a digital camera)... If any glare hits it or any infrared besides this bar are in the way of its path to the Wii mote, it will mess up. Also I believe there is a Distance limit between the two.
If you've ever played with Robbie the Robot in the 1980's then your Nintendo Elite!!!!