Report "Sigma bc500 sensor will not pick up wheel rotation...just received new unit installed according to directions (which aren't very good, just pic diagram). I have the one sensor on the spoke and it is directly across to the stem of the frame, where the other sensor is. The computer is working as far as the clock, etc, but when my son rides the bike, nothing registers, except the clock mode. Any solutions would be wonderful. Thanks. Rosemary from Palm Beach, Florida"