如何用UpatePanel实现省市区的局部刷新

代码如下：

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlProvince" runat="server" AutoPostBack="True"
onselectedindexchanged="ddlProvince_SelectedIndexChanged">
</asp:DropDownList>
<asp:DropDownList ID="ddlCity" runat="server" AutoPostBack="True"
onselectedindexchanged="ddlCity_SelectedIndexChanged">

</asp:DropDownList>
<asp:DropDownList ID="ddlCounty" runat="server">

</asp:DropDownList>

</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ddlProvince"
EventName="SelectedIndexChanged" />
<asp:AsyncPostBackTrigger ControlID="ddlCity"
EventName="SelectedIndexChanged" />
</Triggers>

</asp:UpdatePanel>

但是第一次点下拉框时，不会局部刷新，以后就可以，不知道什么原因,请高手指点