โค้ด vb combobox เลือกจังหวัด อำเภอ แต่ตำบลไม่แสดงครับ ไม่รู้ทำไม

   Private Sub caldistrict()
        Dim conn As New SqlConnection
        Dim strcon As String = "Data Source=KAN-PC\SQLEXPRESS;Initial Catalog=master;Integrated Security=True"

        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.ConnectionString = strcon
        conn.Open()

        If cbprovince.SelectedIndex <> 0 Then
            'If cbprovince.ValueMember = "" Then
            Dim sql As String = "SELECT  * FROM district where id_province = '" + cbprovince.SelectedValue + "' "

            Dim da As New SqlDataAdapter(sql, conn)
            Dim dt As New DataTable
            dt.Clear()
            da.Fill(dt)
            'cbdistrict.Items.Clear()

            Try
                With cbdistrict

                    .DataSource = dt
                    .DisplayMember = dt.Columns.Item("district").ColumnName
                    'คือตัวที่ใช้แสดงใน Combobox
                    .ValueMember = dt.Columns.Item("id_district").ColumnName 'Code หรือรหัสที่เราสร้างขึ้นมาเช่น Primary Key

                End With

            Catch ex As Exception
                cbdistrict.Text = Nothing
            End Try
        End If
        'End If
    End Sub
    Private Sub calsubdistrict()
        Dim conn As New SqlConnection
        Dim strcon As String = "Data Source=KAN-PC\SQLEXPRESS;Initial Catalog=master;Integrated Security=True"

        If conn.State = ConnectionState.Open Then
            conn.Close()
        End If
        conn.ConnectionString = strcon
        conn.Open()

        If cbdistrict.SelectedIndex > 0 Then
            If cbdistrict.ValueMember = "" Then
                Dim sql As String = "SELECT  * FROM subdistrict where id_district = '" + cbdistrict.SelectedValue + "' "
                Dim da As New SqlDataAdapter(sql, conn)
                Dim dt As New DataTable
                dt.Clear()
                da.Fill(dt)


                Try

                    With cbdistrict
                        .DataSource = dt
                        .DisplayMember = dt.Columns.Item("subdistrict").ColumnName
                        'คือตัวที่ใช้แสดงใน Combobox
                        .ValueMember = dt.Columns.Item("id_subdistrict").ColumnName 'Code หรือรหัสที่เราสร้างขึ้นมาเช่น Primary Key

                    End With
                Catch ex As Exception
                    cbsubdistrict.Text = Nothing
                End Try
            End If
        End If
    End Sub


    Private Sub cbprovince_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles cbprovince.SelectedIndexChanged
        caldistrict()
    End Sub

    Private Sub cbdistrict_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles cbdistrict.SelectedIndexChanged
        calsubdistrict()
    End Sub