unity3d - Object not destroying after reaching a certain point Unity -

okay i'm making infinite runner , technique i've used keep player static , move , instantiate platform objects. i've created array list of objects platforms. , i'm adding them in , spawning them. translating them along z-axis , want destroy objects go below 0 in z axis , add object in replacement. problem doesn't translate objects unless add script , doesn't destroy or add if add script translation.

my code follows. if have trouble understanding problem, please ask try explain again.

i have 2 scripts.

1) platformmanager: 1 applied on empty gameobject , contains arraylist.

public class platformmanager : monobehaviour {     [hideininspector]     public list<gameobject> platforms = new list<gameobject>(); // list of platfroms.     public gameobject[] prefab; // allow user add many prefabs through inspactor.      public static float noofplatforms;  // variable hold how many platoforms.     [hideininspector]     public static float objectposition; // z position of game object.     [hideininspector]     public static float objectscale;          // use initialization         void start ()         {         noofplatforms = 6.0f;         objectposition = 0.0f;         objectscale = 0.0f;           platforms.add((gameobject)instantiate(prefab[random.range(0,prefab.length)], new vector3(0,0,0) ,quaternion.identity));         //platforms.add((gameobject)instantiate(prefab[random.range(0, prefab.length)], new vector3(0, 0, 40.40114f), quaternion.identity));         for(int = 0; < noofplatforms; i++){             objectscale = platforms[platforms.count-1].transform.localscale.z;             objectposition = platforms[platforms.count-1].transform.localposition.z;              platforms.add((gameobject)instantiate(prefab[random.range(0,prefab.length)], new vector3(0,0,(objectposition + objectscale)+277f) ,quaternion.identity));         }         }          // update called once per frame         void update ()         {          } } 

2) platformcontroller: 1 supposed instantiate , destroy , translate objects.

public class platformcontroller : monobehaviour {     //private bool canbedestroy = true; //  flag check whether gameobject shoud destroyed or not.     [hideininspector]     public platformmanager manageplateform; //  reference plateformmanager script.      [hideininspector]     public float plateformspeed = 10f;     [hideininspector]     public gameobject allplatforms;     [hideininspector]       //  awake called when script instance being loaded.     void awake()     {         //  accessing plateformmanager script plateform.         manageplateform = gameobject.findgameobjectwithtag("platform").getcomponent<platformmanager>();     }      void start()     {      }       //  update called once per frame     void update()     {          //  first platform object.         gameobject firstplatform = manageplateform.platforms[0];         //  last platform object.         gameobject lastplatform = manageplateform.platforms[manageplateform.platforms.count - 1];          if (firstplatform.transform.localposition.z < 0f)         {             destroy(firstplatform.gameobject); // destroy first plateform gameobject.              manageplateform.platforms.remove(firstplatform); // remove destroyed object list.              // when game object destroyed instantiate 1 gameobject list , add @ last point.             manageplateform.platforms.add((gameobject)instantiate(manageplateform.prefab[random.range(0, manageplateform.prefab.length)], new vector3(0, 0, (lastplatform.transform.localposition.z + lastplatform.transform.localscale.z) + 277f), quaternion.identity));         }         // move available platforms in list along z-axis         foreach (gameobject platform in manageplateform.platforms)         {             platform.transform.translate(0, 0, -plateformspeed * time.deltatime);         }     }  }