<asp:DropDownList ID="ddlTime" runat="server" CausesValidation="true" 
                                        AutoPostBack="true" 
                                        onselectedindexchanged="ddlTime_SelectedIndexChanged">
        <asp:ListItem  Selected Value = "True">--Select--</asp:ListItem>
                                        <asp:ListItem>8.50</asp:ListItem>
                                        <asp:ListItem>9.00</asp:ListItem>
                                        <asp:ListItem>9.50</asp:ListItem>
</asp:DropDownList>

I have drop down with the event changing . but when ever i select an value this event is not firing at all

protected void ddlTime_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

What is the issue that is causing here any help would be great thank you