Hi Paul
Have you tried your Z motor on another port and the Z port with another motor? Also check that the end stops are working by sending a M119 command as you click each one in turn check too that they are plugged into the correct port I.e the switch on the X tower is in the X endstop port. You may also want to check you have your X, Y and Z in the right order. Looking from the front the tower on the left is X (1), the tower on the right is Y (2) and the rear tower is Z (3). You have probably checked all that but going back to basics is the best way to diagnose a fault.
Andy
Have you tried your Z motor on another port and the Z port with another motor? Also check that the end stops are working by sending a M119 command as you click each one in turn check too that they are plugged into the correct port I.e the switch on the X tower is in the X endstop port. You may also want to check you have your X, Y and Z in the right order. Looking from the front the tower on the left is X (1), the tower on the right is Y (2) and the rear tower is Z (3). You have probably checked all that but going back to basics is the best way to diagnose a fault.
Andy