The Voltage written on a modern stepper is usually down around 1 Volt. They don't expect you to use 1 Volt, they expect you to use maybe 36V to overcome the coil inductance and then limit the current so it doesn't go pop.

There was a time, long ago, when people used the L200 stepper driver chip and 6 or 12V steppers were quite normal.

If you can turn the motor with 6V applied then maybe you have coils in series? Even then it doesn't sound at all right.