Endless rendering doesnt work

so i saw a video on how to make a endless runner game and i gave it a try. Everything went perfect untill i made a prefab that i had to place ,and what the prefab is supposed to do is make a random and endless running path. Instead it only makes a path on that position where that prefab is placed. This is my script :

using UnityEngine; using System.Collections;

public class SpawnScript : MonoBehaviour {

 public GameObject[] obj;
 public float spawnMin = 1f;
 public float spawnMax = 2f;

 // Use this for initialization
 void Start () {
     Spawn();
 }

 void Spawn () 
 {
     Instantiate(obj[Random.Range (0, obj.GetLength(0))], transform.position, Quaternion.identity);
     Invoke ("Spawn", Random.Range (spawnMin, spawnMax));
 }

}

This is a link to the video : he is making the script and prefab at 34:30 minutes

I hope someone can help me and understand what i mean ,i know i didnt explain it really good that's because my English isn't that good.