If they are clone boards, you'll need to not measure voltage not at the VREF pin, but the middle pin potentiometer directly.
The people that cloned them screwed up and wired the VREF measurement point to VMOT (+12V) rather than VREF.
Sometimes when wired up correctly, if the current is set too high or too low, the motor wont spin.
The pin-out for the RJ11's is:
(Pin) -> (Motor Coil)
2 -> A
3 -> B
4 -> B
5 -> A
If you wire them up in a way that switches part of A with part of B, (e.g. ABAB or BABA), they won't work properly even if they do spin a little.