I agree with Jazz - if you can't get the current drivers to work without buying anything, get some better drivers like m542 or 80V drivers if the motors can withstand that.

It does sound like you might have a problem with one or more of the motors. Disconnect each motor and measure the resistance between each of the 4 wires (i.e. 6 combinations) and compare it to the Z motor which is working. One of the phases may have burnt out, which would cause the motor to only move very slightly. If you've not got a meter to do this, then you can disconnect the motor from the driver, try turning the motor with each of the two coils/phases short circuited and if it gets harder to turn when short circuited then the motor is probably fine.