Adding TreeView nodes at runtime, but Node count stays at 1

Hi,

I have a treeview control that populates from a database.  I hardcoded a root node, and using PopulateOnDemand I believe this attempts to populate the TreeView when opening the page

The treeview populates OK, without a problem.  However the issue is coming where I want to add another node to one of the nodes added at run time.  It is only counting 1 node (so therefore, I cannot iterate through all the nodes to find the one I'm adding to, as the program believes there is only one node!)

<!--aspx file-->
<asp:TreeView ID="tvTariff" runat="server" OnSelectedNodeChanged="tvTariff_SelectedNodeChanged" OnTreeNodePopulate="tvTariff_TreeNodePopulate" ShowLines="True">
    <Nodes>
        <asp:TreeNode PopulateOnDemand="True" Text="Tariff List" Value="Tariff List"></asp:TreeNode>
    </Nodes>
</asp:TreeView>
 
 
//C#
protected void tvTariff_TreeNodePopulate(object sender, TreeNodeEventArgs e)
{
    if (e.Node.ChildNodes.Count == 0)
    {
        switch (e.Node.Depth)
        {
            case 0:
                //Tariff - root
                PopulateParent(e.Node);
                break;
            case 1:
                //Add Code Group - tariff clicked on
                PopulateCodeGroup(e.Node);
                break;
        }
    }
}
 
void PopulateParent(TreeNode node)
{
    //TreeViewSrc is a class file that returns the data from the database
    TreeViewSrc trSrc = new TreeViewSrc();
    DataTable dtRoot = new DataTable("Tariff");
 
    dtRoot = trSrc.RootNodes();
 
    foreach (DataRow row in dtRoot.Rows)
    {
        TreeNode NewNode = new TreeNode(row["ta_name"].ToString(), row["ta_id"].ToString());
        NewNode.PopulateOnDemand = true;
        NewNode.SelectAction = TreeNodeSelectAction.SelectExpand;
        node.ChildNodes.Add(NewNode);
 
    }
}
 
 
//Code that finds the node, and adds a new child - this is the bit not working, as it only counts 1 node
protected void btnAddCG_Click(object sender, EventArgs e)
{
    int i;
    i = tvTariff.Nodes.Count;
 
    for(i=0;i < tvTariff.Nodes.Count;i++)
    {
        switch (tvTariff.Nodes[i].Depth)
        {
            case 1:
                //only looking for tariff node to add a session node to
                CheckTariffNode(tvTariff.Nodes[i]);
                break;
        }
    }
}
protected void CheckTariffNode(TreeNode node)
{
    if (node.Value==Session["SelectedNode"].ToString())
    {
        TreeNode NewNode = new TreeNode(dlTariffCG.SelectedItem.ToString(),dlTariffCG.SelectedValue.ToString());
        NewNode.PopulateOnDemand = true;
        NewNode.SelectAction = TreeNodeSelectAction.SelectExpand;
 
        node.ChildNodes.Add(NewNode);
    }
}

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