What OS, try going into a room and turning off the lights, as the OS versions go higher the light autosensor changes, mine now will turn the trackball light off if there is enough light. Also try a soft reset then a batt pull, check and make sure the sensor has no obstructions, try taking the housing off air dusting everything, also with the housing off see if the trackball light turns on, the light itself may have died, worse case.
