Yes you are missing steps because you are sending the pulses too fast for the load on the motor.
So simply slow down the speed.
A stepping motor can be made to run faster if the speed is ramped up to the final speed and ramped down before stopping, but this is harder to do.
What you have done is a classic test to see if your motor is being stepped too fast, and yours is.
The other way to get more torque out of a motor is to supply it with a higher voltage, or adjust your driver to give more current.
my motor is nema17
Which only tells you about the size of the mounting of the motor, nothing about the current it takes.