Tree view filtering does not return all the files

<pre>private TreeNode FindNodeByValue(TreeNodeCollection nodes, string searchstring)
    {
       
        // Loop through the tree node collection
        foreach (TreeNode node in nodes)
        {
            // Does the value match the search string?
            if (node.Value.ToUpper().Contains (searchstring.ToUpper()))
                // Yes it does match - return it
                return node;
            else
            {
                // No it does not match - search any child nodes of this node
                TreeNode childNode = SearchChildNodes(node, searchstring);
                // If the childNode is not null it was a match
                if (childNode != null)
                    // Return the matching node
                    return childNode;
            }
        }
        // If the matching node is not found return null
        return null;
    }

    /// <summary>
    /// This method searches a node's ChildNodes collection to find a matching value
    /// with the incoming search string
    /// It will iteratively call itself as it drills into each nodes child nodes (if present)
    /// </summary>
    /// <param name="parentNode">Parent node to search for a match</param>
    /// <param name="searchstring">string to be matched with the Nodes Value property</param>
    /// <returns>Treenode of the matching node if found.  If not found it will be null</returns>
    private TreeNode SearchChildNodes(TreeNode parentNode, string searchstring)
    {
        // Loop through the child nodes of the parentNode passed in
        foreach (TreeNode node in parentNode.ChildNodes)
        {
            // Does the value match the search string?
            if (node.Value.ToUpper().Contains(searchstring.ToUpper()))
                // Yes it does match - return it
                return node;
            else
            {
                // No it does not match - recursively search any child nodes of this node
                TreeNode childNode = SearchChildNodes(node, searchstring);
                // If the childNode is not null it was a match
                if (childNode != null)
                    // Return the matching node
                    return childNode;
            }
        }
        // If the matching node is not found OR if there were no child nodes then return null
        return null;
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        TreeNode trnode=FindNodeByValue(TreeView1.Nodes, fieldFilterTxtBx.Text);
        if (trnode != null)
        {
            TreeView1.Nodes.Clear();
          //  TreeNode newnode = new TreeNode("Detail Engineering");
           // TreeView1.Nodes.Add(newnode);
            TreeView1.Nodes.Add(trnode);
            TreeView1.ExpandAll();
        }
        else

{

    Label1.Text = "No file found";

}
    }