Need a different approach to programming a closed loop stepper motor for a lathe spin

I recently purchased a closed loop stepper ml]closed loop stepper motors)
rated at 12.5nm torque，which came as a set along with the driver and I wrote a pulse program and mapped some buttons with an arduino.
The program and buttons work as I expect and my motor reaches 1360 rpm but if I go above 600 rpm the motor has no torque (I can hold the attachment on the motor shaft with my hand and it stalls).
I would really appreciate some advice and help with a different approach to programming as I am new to all this and my goal is to reach 1000 to 1300 rpm and keep the torque high.

You may be at the frequency limit of the stepper motor. They are usually designed for reasonable torque but not high speed. Remember you have some pretty big coils in there, and they are inductors, and one of the rules in electronics is you cannot change the current through an inductor instantaneously, no matter how hard you try. So what is likely happening is that at the high speed you are never really reaching the full current in the winding, and the force is going to be proportional to the current, so it can't put out very much force which you are perceiving as low torque.

Are you firing it bipolar or unipolar?