You could have a 'broken' cable... Remove it from the wheel and the speedo ends, and it should spin, at the one end, when turning the other.
Also, if you have had your front wheel off lately, be sure that the speedo stop is aligned correctly, and that the set screw is tight.
Black Dog
Careful on set screw, lightly snug or you'll snap the plastic ring. If you do, draw a black zip-tie tightly around and it'll hold as long as it only breaks in one spot.
I love zip ties for all the right reasons and occasionally the right reasons too.
