Stepper driver voltages

[old man emu]

I've just replaced all the stepper drivers on my RAMPS board with Stepstick drivers. Dr Luigi (Professor of Printology) said that the Vref for these stepper drivers is 1.28V. I set all my steppers to this and three of the five motors sang like canaries. The two Z-axis motors didn't move at all. As I still had the Pololu driver, I refitted it and set Vref to something like 0.34V, and then these steppers joined the other song birds.

The Steptsicks were brand new, and selected at random for fitting. Anyone want to hazard a guess why a new driver would not fire up a pair of motors? I had read something about having to double the Vref when driving two steppers off the one driver, but that doesn't make sense because the Pololu I used was set at the Vref for a single motor.

Trying to find detailed information on how these stepper drivers work is near impossible. The manufacturer's site only gives enough information to get the things functioning.

Old Man Emu