Tile generation problem

I have a problem which isn't there in the editor but whenever I build, it is a problem. The problem is that my terrain generation doesn't generate the middle tile. (It is a 3x3 tile system, which creates a procedurally generated terrain based on the position of the player) The tile isn't there which is a problem because the player can then jump out of the world. I have checked that my tile function is called and that the end of the script is called.

EDIT:

 //terrainSize = 500;
 //terrainHeight = 225;
 function UpdateTile(fromnx : int, fromnz : int, nx : int, nz : int) {
 if (Network.isServer) {
     Seed = PlayerPrefs.GetInt("Seed");
 }
 Random.seed = Seed;
 var Chunk : Terrain;
 
 var ChunkPosX : float = terrainSize * nx;
     var ChunkPosZ : float = terrainSize * nz;
 
     var tmp = GameObject.Find("Chunk "+fromnx+","+fromnz);
     if (tmp) {
         Destroy(tmp);
     }
 var td = new TerrainData();
 td.heightmapResolution = terrainSize/2;
     
 td.size = new Vector3(terrainSize, terrainHeight, terrainSize);
 
 var tmp1 = Terrain.CreateTerrainGameObject(td);
 tmp1.tag = "Chunk";
 Chunk = tmp1.GetComponent(Terrain);
     Chunk = tmp.GetComponent(Terrain);
     var hw : float = Chunk.terrainData.heightmapWidth;

     for (var child : Transform in Chunk.transform) {
             if(child.name == "TreeMaster") {
                     GameObject.Destroy(child.gameObject);
                     Debug.Log("Destroying Treemaster ("+nx+","+nz+")");
             }
     }
     var TreeMaster : GameObject = new GameObject();
     TreeMaster.name = "TreeMaster";
     TreeMaster.transform.parent = Chunk.transform;
     Chunk.transform.position = Vector3(ChunkPosX, 0, ChunkPosZ);
     Chunk.name = "Chunk "+nx+","+nz;
     
     var perlin : PerlinNoise;
     var heightmap = new float[hw,hw];
     for(var x = 0; x < hw; x++) {
         for(var z = 0; z < hw; z++) {
             var worldPosX : float = (x+((hw-1)*(nx-1)))*(terrainSize/hw);
             var worldPosZ : float = (z+((hw-1)*(nz-1)))*(terrainSize/hw);
             if (perlin == null) {
                 perlin = new PerlinNoise(Seed);
             }
             var mountains : float = Mathf.Max(0f, (perlin.FractalNoise2D(worldPosX+250, worldPosZ+250, 7, mountainFrq, 0.5f))-0.1);
             var plain : float = perlin.FractalNoise2D(worldPosX-250, worldPosZ-250, Octaves, groundFrq, 0.5f) + 0.5;
             
             var height = (plain + mountains) * terrainHeight;
             heightmap[z,x] = plain+mountains;
             if (TakeItSlow) {
                 if (z % hw == 0 && x % 2 == 0) {
                     yield null;
                 }
             }
         }
     }
     Chunk.terrainData.SetHeights(0,0,heightmap);
     pf++;
 }