Onselectedindexchanged event is not fired

I have add dropdownlist in a usercontrol and that control has been added into a webform, when i select a item, the event is not working
please help me,

This is the code i have used,

   ------------ usercontrol.ascx -------------------
<%@ Control Language="C#" AutoEventWireup="true" Codebehind="usercontrol.ascx.cs" Inherits="TestPortal.Trial._" %>


<asp:DropDownList ID="_dropDownList" runat="server" OnSelectedIndexChanged="_dropDownList_SelectedIndexChanged">
    <asp:ListItem Text="Hindu" Value="1"></asp:ListItem>
    <asp:ListItem Text="Christian" Value="2"></asp:ListItem>
    <asp:ListItem Text="Muslim" Value="3"></asp:ListItem>
</asp:DropDownList>

  & ...