Coroutine for spawning prefab

I'm trying to spawn prefab using coroutine function, but I got a problem with my code. The problem is, the console never goes into the loop functions I have setup for spawn the prefab repeatedly side by side in a certain amount. The result i got just a single object. Here is my code:

     void Start()
     {
         spawnRotation = Quaternion.identity;
         TreeSpawn_Threshold = 40;
         ObstacleSpawn_Threshold = 40;
         FloatingGroundSpawn_Threshold = 35;
         EggSpawm_Threshold = 70;
         RiverSpawn_Threshold = 30;
         PlatformWidth = GameObject.Find("GroundA (5)").GetComponent<BoxCollider2D>().bounds.size.x;
        
       //   InvokeRepeating("GameRun", 1f, 1f);
 
     }
 
     // Update is called once per frame
     void Update()
     {
 
         StartCoroutine(GameRun());
 
 
     }
  IEnumerator GameRun()
     {
 
 
         if (transform.position.x < groundspawnpoint.position.x)
         {
 
                **.....some code**
             //+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
             //spawn floating ground
 
 
             if (Random.Range(0, 100) < FloatingGroundSpawn_Threshold)
             {
                 FloatWidth = 0;
                 LastFloatSpawnPosition = transform.position.x;
                 numfloatground = Random.Range(0, 4);
 
                 for (int i = 0; i <= numfloatground; i++)  **// THIS LOOP FUNCTION NEVER WORK**
                 {
                      **BUT THE CODE BELOW WORK JUST LIKE NO LOOP FUNCTION**
                     //floating ground position
                     Vector3 FloatSpawnPosition = new Vector3(LastFloatSpawnPosition + FloatWidth, floatinggrooundspawnpoint.position.y, floatinggrooundspawnpoint.position.z);
 
                     GameObject floatingGroundOBJ = FloatingGroundPooler.GetPooledObject();
                     floatingGroundOBJ.transform.position = FloatSpawnPosition;
                     floatingGroundOBJ.transform.rotation = spawnRotation;
                     floatingGroundOBJ.SetActive(true);
                     Debug.Log(i);
                     //floating distances width
                     if (numfloatground > 0)
                     {
                         LastFloatSpawnPosition = FloatSpawnPosition.x;
                         FloatWidth = floatingGroundOBJ.GetComponent<BoxCollider2D>().bounds.size.x;
                     }
 
 
 
 
                 }
 
             }
 **.....some code**
 
 
             yield return new WaitForSeconds(1f);
         }
 
         
 
     }
 
 
 
 

I also have a question, why the coroutine can't works when i put the in the Start function?

Thanks in Advance