You have three elements, stepper, wiring and motor and need to figure out which one is wrong.
Install UGS 1.0.9. so you can jog each axis individually.
Swap the motor with one of the motor which is running fine, this will tell you if you motor is ok
Attach a good running motor to your Z axis leads, if it does not run maybe a faulty connection
Attach a good runnin motor directly to the stepper driver without the leads, if it runs there is a problem with the leads.
Verify the amps of the stepper driver, if it is ok but still not running the chance is big it is defective, call inventables and explain your situation