If steppers have no label, you can still make sure what are they if you measure their coil resistance with a digital multimeter. Between 30 ohms and 1.65ohms the difference will be hard to miss.
The values match each other so its either 12v/0.4a=30ohms or its the good type of 2.8v/1.68a=1.66ohms. The latter is much preferable to the first. If you have 12v ones and you can change them easily - then do so asap.
If you have 12v psu and 30ohms coil the current cant get past the point indicated by ohms law, even if you set driver to more. In that case makes sense for Z to not respond to increase in driver setting. To fix Z, lets hope the motors coils are currently in series, and then perhaps you could try putting them in parallel, hopefully that would fix Z.
The values match each other so its either 12v/0.4a=30ohms or its the good type of 2.8v/1.68a=1.66ohms. The latter is much preferable to the first. If you have 12v ones and you can change them easily - then do so asap.
If you have 12v psu and 30ohms coil the current cant get past the point indicated by ohms law, even if you set driver to more. In that case makes sense for Z to not respond to increase in driver setting. To fix Z, lets hope the motors coils are currently in series, and then perhaps you could try putting them in parallel, hopefully that would fix Z.