Need help increasing the torque on my Nema 17 Stepper motor

[samlevy0515]

I am very new to all of this but I am trying to create a robot to turn a Rubik's cube automatically and currently the motors are having trouble turning the sides of the cube. The Nema 17 stepper motor is currently connected to a 12V 2A power supply and it seems to keep slipping no matter what I try. If i give even a little bit of force to help it begin the turn it can then complete the turn with no issues. I have attached the code I am running and a copy of the motor's datasheet and a very rough diagram of my setup. Any advice/things I could try to increase the torque of the motor would be greatly appreciated!