First, check that you have not inadvertently set the Z motor stepper current too low with the M906 command that you probably have in config.g.
Next, you need to establish whether it is the motor, the driver on the Duet board, or the wiring in between that is the problem. You can do this by swapping motors and/or connections. You can also test the stepper windings and the wiring loom with a multimeter.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
