Stepper motor stalls while accelerating

Hello guys,

This is my second attempt to run stepper motor using a driver I made myself.

The first attempt was running the stepper motor with a constant speed and it was working fine.

the second one -which I'm doing now- is accelerating the motor speed with a ramp profile, but the motor begins to lose steps and finally stops!! Attached is a video of when this is happening.
The driver is 4 TIP122 NPN transistors driven by 4 2n2222 NPN transistors, which are isolated using opto-couplers from the microcontroller.
The motor is unipolar stepper.

please help me with that problem.

You might try reducing the acceleration until you stop losing steps.

Try decreasing the power resistors by 10% ---- it will draw a little more current but should increase speed