Trouble ramping up stepper motor
The relatively high inductance of stepper motor windings means that as you increase the step speed for a given supply voltage, the current (and hence torque) produced falls off dramatically. Basically what you've done is step it at such a high frequency that the current in the winding cannot build to a point where enough torque is produced to overcome the magnetic detent force.

To fix this you need to increase the supply voltage to the motor when it is operating at the higher speeds.

Also note that the motor will become extremely inefficient at high speeds due to the large hysteresis/eddy current losses in the iron core from the high switching speeds. It is probably much better to either use a gearbox or some other type of motor if you want to run it above Fs.