A stationary stepping motor provides the maximum torque.
As the speed of movement increases the torque it can produce drops. It will eventually reach a point where the internal friction and inertia of the motor stop it from moving.
So if you want more torque then slow down the speed of movement you are using.
This is not a broken motor, it is unrealistic expectations of what a stepping motor can do.
You can increase the speed by:-
-
supplying more current. But do not exceed the rated current of the motor or driver.
-
increasing the voltage of the drive, so that the current gets into the coils faster. But keep within the voltage ratings of your driver. This will require a different power supply.