Hashtable Bejeweled movement

I have a field with cubes generated from a hash table like bejeweled. When i click a cube, the cube is being removed from the hash table and i remove the object.

I want the cubes above the just removed cube to move down.

This is the code i have:

My code is a bit of a mess because i couldn't remove a key from the hash table in a for each loop. The method remove adds all the id's from the selected cubes to a array list. after that is done the Removegroup function removes the cubes from the hash table.

After that the MoveDown function is called. It adds all the cubes without a cube underneath (exept the bottom row) to a array lists and the method remove, moves all that cubes a position down and calls MoveDown again.

I think the problem is in the Remove function (renaming the cubes position in the hash table)

public Hashtable cubes;

     public void Remove ()
 {
     verwijderGroep.Clear ();
     foreach (string id in cubes.Keys) 
     {
         GameObject blokje = (GameObject)cubes[id];
         if (blokje.GetComponent<Cube> ().isSelected == true)
         {
             verwijderGroep.Add (id);
             Destroy (blokje);
         }
     }
     RemoveGroup (verwijderGroep);
 }
 
 public void RemoveGroup (IEnumerable verwijder)
 {
     foreach (string id in verwijder)
     {
         cubes.Remove (id);
     }    
 }

 public void Verplaats ()
 {
     foreach (string id in verwijder) 
     {
         blokje2 = (GameObject)cubes[id];
         Vector3 underCube = new Vector3 (blokje2.transform.position.x, blokje2.transform.position.y - 1, blokje2.transform.position.z);
         cubes.Remove (id);
         cubes.Add (underCube.ToString (), blokje2);
         
         iTween.MoveTo (blokje2, new Vector3 (blokje2.transform.position.x, blokje2.transform.position.y - 1, 0), 1);    
     }
     MoveDown ();
 }
 
 
 public int nogverkeerd = 0;

 public void MoveDown ()
 {
     nogverkeerd = 0;
     verwijder.Clear ();
     foreach (string id in cubes.Keys) 
     {
         GameObject blokje = (GameObject)cubes[id];
         Vector3 underCube = new Vector3 (blokje.transform.position.x, blokje.transform.position.y - 1, blokje.transform.position.z);
         if (!cubes.Contains (underCube.ToString ()) && blokje.transform.position.y != 0.0) 
         {
             verwijder.Add (id);
             nogverkeerd = 1;
             //Debug.Log (id);
         }
     }
     if (nogverkeerd == 1)
         Verplaats ();
 }

Thanks