I have NEMA 17 steppers
That tell us nothing useful about the motor apart from the size of the mounting.
and adjusted the voltage on the A4988 for 0.62 volts
I thought you adjusted them for current not voltage. No wonder it does not perform if the output voltage is 0.62 volts. Unless you meant reference voltage, but you did not say. Have you read the instructions?
Is there a sketch that will allow me to test the individual A4988s on a breadboard?
Bread boards are most unsuitable for carrying such large currents.
You keep us short of relevant information, what libraries are you using?
Look under the File-> Examples menu for that libiary.