Refresh combobox after data is added

Hi experts -
I am using flex builder 2.  I have a page with a combobox, the dataProvider is an array collection that gets populated from a database; when you double-click the combo box it opens a pop up.  You then enter the name of a new supplier in the pop up and it adds it to the database.  Next the pop up closes and the webservice is called to re-populate the ArrayCollection.  I am trying to get the combo-box to refresh so it shows the new data, but it won't.  I checked to see that the data is added correctly but it wont show up in the list until you completely close the app and re-open it again.  I have attached the code for the combobox and the code that I am using to re-populate the Array Collection.

<mx:Binding source="cboSupplier.dataProvider as ArrayCollection" destination="obj.SupplierDepartmentTypes"/>
 
<mx:ComboBox x="191" y="30" id="cboSupplier" dataProvider="{obj.SupplierDepartmentTypes}" labelField="Value" width="200" 
				doubleClickEnabled="true" doubleClick="this.supplierDepartmentDoubleClicked(KeyPair(cboSupplier.selectedItem).Name);" 
				change="obj.UpdateRmr(Program.currentUser, 'Vendor_Dept', String(KeyPair(cboSupplier.selectedItem).Name), obj.myCursor.current.RMRNum, 0);">
			</mx:ComboBox>
 
public function supplierDepartmentDoubleClicked(supplierDepartmentType:String):void
    {
 
            var popup:NewSupplierDepartmentDialog = PopUpManager.createPopUp(this,NewSupplierDepartmentDialog,true) as NewSupplierDepartmentDialog;
            
            PopUpManager.centerPopUp(popup);
            //check to see if there is a listener for pop-up manager
            //obj.GetSuppliers();
    }
 
public function GetSuppliers():void
		{
			var ws:WebService = Program.m_webServiceManager.getWebservice();
			ws.GetSupplierDepartments.addEventListener(ResultEvent.RESULT,LoadSuppliers);
			ws.GetSupplierDepartments();
		}
		public function LoadSuppliers(e:ResultEvent):void
		{	
			var dataSet:DataSet = new DataSet(String(e.result));
        	if(dataSet.HasErrors())
        	{
        		Alert.show(dataSet.GetErrorAt(0));
        	}
        	else
        	{
        		this.SupplierDepartmentTypes=new ArrayCollection;
        		var dataTable:DataTable = dataSet.GetTableAt(0);
        		var keyPair:KeyPair = new KeyPair();
        		
        		keyPair.Name = "0";
	        	keyPair.Value = "Select Supplier"; 
	        	this.SupplierDepartmentTypes.addItem(keyPair);
	        		        	
        		for(var index:int=0;index<dataTable.RowCount();index++)
        		{
        			var row:DataRow = dataTable.GetRowAt(index);
        			keyPair = new KeyPair();
	        		keyPair.Name = String(row.GetValue(0));
	        		keyPair.Value = String(row.GetValue(1)); 
	        		this.SupplierDepartmentTypes.addItem(keyPair);
        		}
        		//this.SupplierDepartmentTypes.enableAutoUpdate();
        		SupplierDepartmentTypes.refresh();        		
        	}
        	
		}

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