Stepper Motor does not move with Mb.run ()

[LolloMazzy]

Why my stepper motor doesn't move with the instruction Mb.run() every loop, instead with this delay it can?

void loop() {
if ((millis() - updateMB) >= 50) {
Mb.Run(); // funzione per update dati da/a ModBus
updateMB = millis();
}
stepper.setSpeed(vel);
stepper.runSpeed();
}