Sounds as if the drivers are not registering all of the step pulses the controller is sending them. What pulse frequency and pulse width are you sending them? You might try to slow down the frequency to give them a chance.

I found that I had to slow the frequency down to 100kHz before the Chinesium stepper drive I was playing with would work consistently. Apparently that is about right for many of these things. The fact it used to work but now sort of doesn't suggests that you were marginal to be begin with. That might have required only slight changes in things like the wiring, temperature and age of the components to mess things up.

I'd suggest you lower the stepper frequency (in the controller setings) as a first step and see where that gets you.