I'm also having a problem slaving the A-axis with the Y-axis.
My original driver failed and upon replacing it noticed that it had been set-up with two stepper motors (Y-axis) being driven by a single driver. Some research identified that each driver should have a dedicated driver, therefore I
replaced the single driver with 2 new drivers and re-wired.
In Mach 3 I selected Slave Axis and slaved 'A' to 'Y', then re-started Mach 3 as instructed
I have ensured that motor outputs are enabled for the A-axis, the motor tuning is identical for both motors and the A-axis is not inhibited (in Settings)
I have tested the stepper motors and each works so there is unlikely to be an issue with the wiring or drivers
I cannot seem to make the slave (A-axis) stepper motor work, i.e. when I select and jog the Y-axis in Mach 3 only one motor turns. Maybe the Motor Output numbers (Step Pin and Dir Pin) for the A-axis are incorrect?
Any help would be massively appreciated...