Why doesn't the Arduino motor do what it should?

Hello, this is my third question on the Arduino topic.

I used this sketch:

const int forwardPin = 8;

const int backwardPin = 12;

const int delayTime = 2000;

void setup(){

pinMode(forwardPin, OUTPUT);

pinMode(backwardPin, OUTPUT);

}

void loop(){

digitalWrite(forwardPin, LOW);

digitalWrite(backwardPin, LOW);

delay(delayTime);

digitalWrite(forwardPin,HIGH);

digitalWrite(backwardPin,LOW);

delay(5000);

}

and I'm just experimenting a little bit so I know how the engine works.

However, it doesn't do what it's supposed to. Sometimes it just rotates jerkily, then for 10 seconds, then not at all, and then it rotates until I unplug it, etc.

I don't know what the problem is and hope someone can answer this for me so that the engine finally does what it's supposed to.

Here are some more pictures of my setup: