The coil current limit must be properly set before the motor is used. That means no guessing. Get the coil current limit from the motor data sheet or data plate. Use the instructions to set vref carefully.
Setting coil current:
You do not have to set Vref (coil current limit) at max. Set it high enough that the the motors perform adequately without missing steps.
Stepper basics:
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The DRV8825 drivers are good for 1.5A with no heat sink and 2.2A with heatsinks and active (fan) cooling. What are the specs of your steppers? Please post a data sheet for the motor.