I can't get an UpdatePanel on my page to work.  One control - an
ImageButton in a GridView - works the first time you click on it, but
then freezes all of the controls on the form.  The paging controls of
the GridView refresh the entire page, as if the UpdatePanel wasn't
there.  There is an outside control - a dropdownlist - which is set to
trigger the updatepanel, and also refreshes the entire page.  Below
are some snippets from the code.  Any ideas?

<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server" />
...
<asp:DropDownList ID="ChildSites" runat="server" AutoPostBack="True"
OnSelectedIndexChanged="ChildSites_SelectedIndexChanged"></
asp:DropDownList>
...
<asp:UpdatePanel ID="MainUpdatePanel" runat="server"
updatemode="Conditional" childrenastriggers="true">
 <Triggers>
  <asp:AsyncPostBackTrigger ControlID="ChildSites" />
 </Triggers>
<ContentTemplate>
 <div >
 <div> Total<img src="star_active.png" alt="Approved" />s: <asp:Label
ID="TotalStarsLabel" runat="server" /></div>
 </div>
 <asp:GridView ID="AnnouncementsGrid" runat="server"
enablepaging="true" PageSize="10"
OnRowDataBound="AnnouncementsGrid_ItemDataBound">
  <Columns>
   <asp:TemplateField>
    <ItemTemplate>
     <asp:ImageButton ID="ApprovalStatusImage" runat="server"
ImageUrl="star_inactive.png" OnClick="ApprovalStatus_Clicked" />
    </ItemTemplate>
   </asp:TemplateField>
  </Columns>
 </asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>