Флажок не срабатывает в Gridview с сообщением javascript

<asp:GridView ID = "GridView1" runat = "server" AutoGenerateColumns = "false" CellSpacing = "0" GridLines = "None" PageSize = "20"
ShowStatusBar = "false" ShowFooter = "False" ShowHeader = "false" DataKeyNames = "RowType, QID"
AllowFilteringByColumn = "false" AllowPaging = "false" AllowSorting = "false" ShowGroupPanel = "false" Skin = "" Width = "100%"
CssClass = "GVResult" SelectedItemStyle-CssClass = "SelectedStyle" RenderMode = "Lightweight"
OnRowDataBound = "GV_ItemDataBound" OnRowCommand = "GV_RowCommand">
<Columns>
    <asp:TemplateField Visible = "true" ItemStyle-HorizontalAlign = "Left" HeaderStyle-Width = "10%">
        <ItemTemplate>
            <asp:Label ID = "lblQuestionNo" Text='<%# Eval("QuestionNo") %>' runat = "Server" CssClass='<%# Eval("CSS") %>' Enabled = "False" />
            <asp:HiddenField ID = "hfQID" runat = "Server" Value='<%# Eval("QID") %>' />
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField Visible = "true" ItemStyle-HorizontalAlign = "Left" HeaderStyle-Width = "10%">
        <ItemTemplate>
            <asp:Label ID = "lblMarker1" Text='<%# Eval("Marker1") %>' runat = "Server" CssClass='<%# Eval("CSS") %>' Enabled = "true" />
            <asp:CheckBox ID = "ckMarker1" Checked='<%# Eval("CheckMarker1") %>' runat = "Server" AutoPostBack = "true" OnCheckedChanged = "ckMarker_CheckedChanged" />
        </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField Visible = "true" ItemStyle-HorizontalAlign = "Left" HeaderStyle-Width = "10%">
        <ItemTemplate>
            <asp:Label ID = "lblMarker2" Text='<%# Eval("Marker2") %>' runat = "Server" CssClass = "Label_100_White" Enabled = "False" />
            <asp:CheckBox ID = "ckMarker2" Checked='<%# Eval("CheckMarker2") %>' runat = "Server" Enabled = "true" AutoPostBack = "true" OnCheckedChanged = "ckMarker_CheckedChanged" />
        </ItemTemplate>
    </asp:TemplateField>
</Columns>
</asp:GridView>

protected void GV_ItemDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row is GridViewRow & e.Row.RowType == DataControlRowType.DataRow)
        {
            GridViewRow Item = e.Row as GridViewRow;

            CheckBox ckMarker1 = (CheckBox)e.Row.FindControl("ckMarker1");
            CheckBox ckMarker2 = (CheckBox)e.Row.FindControl("ckMarker2");

	ckMarker1.Attributes.Add("onclick", "javascript: var answer = confirm('Do you want to proceed with the changes');return answer;");	
	ckMarker2.Attributes.Add("onclick", "javascript: var answer = confirm('Do you want to proceed with the changes');return answer;");            }
    }

protected void ckMarker_CheckedChanged(object sender, EventArgs e)
{
    DTO.FinalMarkingUpdate f = new DTO.FinalMarkingUpdate();
    BLL.AllocMQuestion F = new BLL.AllocMQuestion();
    CheckBox ck = (CheckBox)sender;
    GridViewRow Item = (GridViewRow)ck.NamingContainer;

    f.OrgID = this.OrgID;
    f.AssessID = this.AssessID;
    f.MarkerNo = int.Parse(ck.ID.Substring(8, 1));
    f.QID = int.Parse(((HiddenField)Item.FindControl("hfQID")).Value);
    f.IsMarker = ck.Checked;
    F.UpdateMarkerAndChecker(f);

    DataSource();
}