The header of UnboundCheckBoxField doesn't respond

I have a webdatagridwhich has some columns. One of them is an UnboundCheckBoxField. My Problem is that the header of the checkboxes is being ignored. However I can find no reason, so I can't check all of the checkboxes at once.

The initialisation in code-behind looks as the following:

        var checkboxColumn = new UnboundCheckBoxField
        {

            Key = "Selected",
            Width = Unit.Pixel(30),
            HeaderCheckBoxMode = CheckBoxMode.BiState,
            Header =
            {
                CssClass = "checkboxHeader",

            },

            HeaderChecked = false,

        };

        SelectWebDataGrid.Columns.Add(checkboxColumn);

On the aspx page, the relevant part for this querstion looks as the following:

ig:WebDataGrid CssClass="WebDataGrid" ID="SelectWebDataGrid" AutoGenerateColumns="false" runat="server"
            OnRowSelectionChanged="RowSelectionChanged" OnInitializeRow="InitializeRow">
            <Behaviors>
                <ig:Activation Enabled="true" />
                <ig:Selection CellClickAction="Row" RowSelectType="Single">
                    <AutoPostBackFlags RowSelectionChanged="true" />
                </ig:Selection>
                <ig:Paging Enabled="true" PagerAppearance="Bottom" QuickPages="2" PagerMode="NumericFirstLast" />
                <ig:Sorting Enabled="true" />
                <ig:RowSelectors RowNumbering="false" Enabled="false" />
                <ig:ColumnMoving Enabled="true" />
                <ig:ColumnResizing Enabled="true" />
                <ig:EditingCore>
                    <Behaviors>
                        <ig:CellEditing Enabled="false" />
                    </Behaviors>
                </ig:EditingCore>
            </Behaviors>
            <ClientEvents AJAXResponse="OnGridAjaxResponse" />
        </ig:WebDataGrid>