what else could cause an odometer to not work

my odometer quit working,  speedometer works fine,  I took it apart and
found a broken plastic little gear,  called a repair place, they said bring it
down to match it up which I did.  The owner told me to make sure the
broken piece was not in the unit as it could get into the works and prevent
them from turning. I inspected and could not see it or any other debris and
assembled and installed, took it out for a test drive and the speedometer
still works but not the odometer what else could it be ? I did check all the
fuses (with a meter) and they were all good.