das sieht danach aus, das der Stepper der vorgegeben schrittfolge nicht nachkommen kann? Nur eine vermutung...

"Well, I think you should leave the delays in your code, to make sure the A4988 recognizes the step pulses. To be on the safe side, I'd put at least a 10 microsecond delay betweeen the step pin state changes. Actually, a long delay at the end of the loop (say half a second or so) will allow you to see each step the motor takes (just like in your previous setup)."