Checkbox doesn't disable on SelectedIndexChange in gridview

I have a gridview that is in an update panel.   At the top of the page is a radiobutton list, that when the selectedIndex Changes, it loops thru and should disable a checkbox field in the gridview.   I've set breakpoints, and it does loop thru, but the checkbox Never disables.  What am I doing wrong?   Is this because it's in an update panel?  If that's the case, how do I get the page to loop thru and disable the checkbox? 
My Radio Button LIst <asp:RadioButtonList ID="RadioButtonList" runat="server" AutoPostBack="True" onselectedindexchanged="RadioButtonList_SelectedIndexChanged"> <asp:ListItem Value="1">Hi</asp:ListItem> <asp:ListItem Value="2">World</asp:ListItem> </asp:RadioButtonList> My template field from my gridview <asp:TemplateField> <ItemTemplate> <asp:CheckBox ID="cbEmail" runat="server" Text="Send e-ma ...