Dropdown In .net Usercontrol Not Maintaining State

This is probably something fundamental and stupid that I've missed, but the various workarounds are causing me such huge headaches that really, I need to understand the basic problem here in order to find the best solution. I have a usercontrol which contains a drop-down list.

<asp:DropDownList ID="ddlOrderStatus" AutoPostBack="true" runat="server" CssClass="textbox">
<asp:ListItem value="8">Pending</asp:ListItem>
<asp:ListItem value="9">On Hold</asp:ListItem>
<asp:ListItem Value="11">Complete</asp:ListItem>
<asp:ListItem Value="12">Cancelled</asp:ListItem>
</asp:DropDownList>

It does nothing in its Page_Load event. This is in turn contained in a page which, in it's Page_Load event does some databinding on some repeater controls on the page but doesn't touch the control containing the ddl. There is no ajax on the page. The dropdownlist is - clearly - set to autopostback = true. The postback code looks like this:

Private Sub ddlOrderStatus_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlOrderStatus.SelectedIndexChanged
Dim ddl As DropDownList = DirectCast(sender, DropDownList)
Dim currentsorting As String = Request.QueryString("sort")
Dim currentpaging As String = Request.QueryString("page")
Response.Redirect(String.Format("~/admin/orders/Manage/0?sort={0}&page={1}&status={2}", currentsorting, currentpaging, ddl.SelectedValue))
End Sub................................