Z motor humming instead of moving down

I have a Velleman K8200. I recently replaced the CPU board and have now realised that the z axis appears not to be working properly. I am not sure if this is a coincidence, and the x and y axis are fine. It seems to be fine moving up (i.e. increasing in Z) but sometimes when I move down (e.g. go from Z 150 to Z 50) the motor just hums. When I disconnected the motor to see if was the cause, the motor seems to do the same thing. I therefore ordered a new motor but even with the new motor the problem persists. All I know is that if I turn the shaft of the motor that sometimes the motor can start rotating again, which I find strange.

Since you just replaced the control board try swapping the driver from the old board to the new board for the Z axis.
Also set the reference voltage to 0.55 for all of them

Thank you but I am not sure what you mean. I communicate directly with the board using G-code. My understanding is that my CPU board and motor are new so I am not sure why they would not work with each other.

You may have a defective driver for the Z axis (it happens)
If you take one of the other drivers off of the old control board and swap it it might get you going again.