Can't load prefabs from resources into an array

I'm making a game with very small levels (think Wario Ware Style). I decided that every microlevel should be a prefab. So I'm going to put all the level prefabs into a Levels directory inside my Resources folder. Then I'm planning to load all the levels into an array, shuffle it with a simple algorithm and then spawn one of them at a time. (I've got an external manager calling the spawn function when needed). Problem is, it seems not to load my levels at all. Is there something wrong in what I'm doing? And plus, is this a good approach? Thank you!

     private Transform _currentLevel;
     private Transform _player;
     private Transform[] _levels;
     private int _levelIndex;
     
     // Use this for initialization
     void Start () {
         _player = GameObject.FindGameObjectWithTag("Player").transform;
         //load all levels from resources
         _levels = Resources.LoadAll("Levels") as Transform[];
         _levelIndex=0;
     }
     
     public void SpawnLevel ()
     {
         if (_levelIndex==_levels.Length-1)
         {
             _levelIndex=0;
             MixLevels ();
         }
         
         _currentLevel=Instantiate(_levels[_levelIndex],transform.position,Quaternion.identity) as Transform;
         _currentLevel.parent=this.transform;
         _player.position=new Vector3(0,1,0);
         _player.gameObject.SetActive(true);
         
         _levelIndex++;
     }
     
     public void DestroyLevel ()
     {
         _player.gameObject.SetActive(false);
         if (_currentLevel) Destroy(_currentLevel.gameObject);
     }
     
     public void MixLevels()
     {
         //apply a Fisher-Yates algorithm to create a new level pool
         for (int i = _levels.Length-1; i>0; i--)
         {
             var r = Random.Range(0,i);
             var tmp = _levels[i];
             _levels[i]=_levels[r];
             _levels[r]=tmp;
         }
     }
 }