dropdownlist in updatepanel post back problem

Howzzz things,

am using 2010 VS, i have Main Master page my App with Forms Authentication. In a Folder called Report i have wedform, web.config file for the folder. In folder it has web form it contains Dropdownlist contol in Updatepanel like this.

<asp:UpdatePanel ID="upCircles" UpdateMode="Conditional" runat="server">

<ContentTemplate>
<asp:DropDownList ID="ddlCircles" 
style="position:absolute; top: 8px; left: 139px; height: 30px; width: 167px;" 
runat="server" AutoPostBack="True" 
onselectedindexchanged="ddlCircles_SelectedIndexChanged">
</asp:DropDownList>
</ContentTemplate>
<Triggers>
</Triggers>
</asp:UpdatePanel>

which will not do post back. But when i take off authorization tag in web.config of report folder it does post back. Any reason why it is not working with forms Authentication. Plz reply thx...