It is the current that needs to be set correct. The voltage to the DRV8825 just needs to be high enough. What is the current rating of the stepper motors, and how high do you have the stepper motor current turned up on the DRV8825 boards?
You may have the current set too low on the DRV8825 boards, or set high enough but the boards are not sufficiently well cooled. Or you may have the acceleration set too high in the firmware. Or you may be using high-inductance stepper motors that require a higher supply voltage to the DRV8825 than you are using, to achieve the required current at high stepping rates.
You may have the current set too low on the DRV8825 boards, or set high enough but the boards are not sufficiently well cooled. Or you may have the acceleration set too high in the firmware. Or you may be using high-inductance stepper motors that require a higher supply voltage to the DRV8825 than you are using, to achieve the required current at high stepping rates.