Drop down list does not fire postback?

Question

Hi friend...in my page contains drop downlist,i have set auto postback property="True" but it does not firing while selected index changed...Can anyone help me out..

Source:

<div class="sel_service">
    <asp:DropDownList ID="ddlRequestType" AutoPostBack="True" runat="server"
        onselectedindexchanged="ddl1_SelectedIndexChanged"></asp:DropDownList>
    </div>

<div class="sel_service">
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" ValidationGroup="Add" runat="server"
        ErrorMessage="Select Request Type" ControlToValidate="ddlRequestType"
        Display="None"></asp:RequiredFieldValidator>
  <asp:ValidatorCalloutExtender ID="RequiredFieldValidator1_ValidatorCalloutExtender"
        runat="server" Enabled="True" TargetControlID="RequiredFieldValidator1">
    </asp:ValidatorCalloutExtender>
    </div>