C Driver not fully working

Hi, I wrote a driver in C for a stepper motor and it's working beautifully with one stepper motor but horribly with another one (same manufacturer, similar characteristics). I've adjusted interval between phase changes with no greater effects. The motor sometimes moves correctly, sometimes jumps around, changes direction and sometimes just vibrates in place. I've checked wiring and they are both exactly the same. I have both pairs of coils (orange-blue & red-yellow, not using white/black) set to the same conector pins. Worst case scenario is one pair is inverted (orange-blue vs blue-orange), which as far as I know would only invert rotation. I've actually tested all 8 combinations (AB CD, BA CD ... CD A and none of them work.

The one that works is a "Sanyo Denki 103H7123-0710"
The one that doesn't is a "Sanyo Denki 103-547-52500"

Can I check anything else? I know the motor is working fine since it's working with a previous hardware (for which I don't have the firmware source)

Thank you for your help!

Assuming that you have both coils connected correctly, then the only thing that can go wrong is that the motor has insufficient torque for the step rate (including acceleration). You can either increase the current or reduce the step rate.

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