Have you tested it off of a 6 v battery?
If it does not run it is not a 6 v motor.

Voltage reducers are essentially a resister.
All not only reduces the voltage it also reduces
the current flow (amps).

6 volt system components need more amps to
function. This is why 6 volt wiring is made with
heavier gauge (thicker) wire.

So when you get the motor running on 12 v then
adding the reducer it keeps running because the
motor has enough momentum to keep moving
with reduced current flow. Indicates that you have
a 12 v motor. In the absence of you testing the motor
on a 6 v battery. Jeffrey