When did it last work properly?
What has changed since then?
You've done half the diagnosis work, proving that tour Z motor works on an alternate driver, now you need to try your Z driver on another motor to definitively isolate the problem to just your Z driver.
You don't give any details of your hardware, if I had this fault on my system I would swap out a driver but this is not an option on "All in One" Break Out Board and driver units where one failed driver renders the whole board scrap.