Same GridView on postback

I am trying to apply a filter to a gridview through a dropdown menu in a header template.  However, after the option is selected, the page reloads and nothing has happened.  I don't know why nothing is happening.  I've tried many things and no filter came through.  Can someone help me?

[code]        <asp:GridView ID="GridView1" runat="server"             AllowPaging="false"             AllowSorting="true"             AutoGenerateColumns="false"             BorderStyle="None"             CellPadding="3"             CssClass="inboundTbl"             EnableModelValidation="True"             PageSize="20"             OnRowDataBound="GridView1_RowDataBound"             OnPageIndexChanging = "OnPaging"             Width="100%"             >[/code]

[code]<asp:TemplateField HeaderText="Current Terminal ID" SortExpression="VUCTID">                     <HeaderTemplate>                         Terminal:                         <asp:DropDownList ID="ddlTerminal" runat="server"                         AppendDataBoundItems = "true"                         AutoPostBack="true"                         OnSelectedIndexChanged="TerminalChanged"                         >                         <asp:ListItem Text="ALL" Value="ALL"></asp:ListItem>                        </asp:DropDownList>                     </HeaderTemplate>                     <ItemTemplate>                         <%# Eval("VUCTID") %>                     </ItemTemplate>                 </asp:TemplateField>[/code]

    private void BindGridView()
    {
        string query = @"This space was intentionally left blank.";
        using (iDB2Connection conn = new iDB2Connection(connect))
        {
            using (iDB2Command cmd = new iDB2Command(query, conn))
            {
                conn.Open();
                iDB2DataAdapter da = new iDB2DataAdapter(cmd);
                DataSet ds = new DataSet();
                da.Fill(ds);
                DataTable dt = ds.Tables[0];
                GridView1.DataSource = ds;
                //iDB2DataReader reader;
                //reader = cmd.ExecuteReader();
                //GridView1.DataSource = reader;
                GridView1.DataBind();
                GridView1.GridLines = GridLines.Horizontal;
                DropDownList ddlTerminal = (DropDownList)GridView1.HeaderRow.FindControl("ddlTerminal");
                this.BindTerminalList(ddlTerminal);
                string[] keys = new string[1];
                keys[0] = "VUUNIT";
                GridView1.DataKeyNames = keys;
                ViewState["tab"] = ds.Tables[0];
            }
        }
    }

    protected void TerminalChanged(object sender, EventArgs e)
    {
        DropDownList ddlTerminal = (DropDownList)sender;
        ViewState["Filter"] = ddlTerminal.SelectedValue;
        this.BindGridView();
    }

    protected void OnPaging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        this.BindGridView();
    }