Commented Unassigned: Dynamic Extender Doing Nothing [27994]

Hi - I'm making a simple page to test DynamicPopulateExtender. Below is my markup. No errors are reported, no exceptions, nothing. Nothing happens to the label field. The web service does not get called. Also tried using a page method. Nothing.

VS2015, latest ACT downloaded. Any clues?

Thanks

<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">
<p>
<asp:CheckBox ID="CheckBox1" runat="server" Text="Do Heavy Calc" OnCheckedChanged="CheckBox1_CheckedChanged" AutoPostBack="true" />
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
<ajaxToolkit:DynamicPopulateExtender ID="DynamicPopulateExtender1" runat="server" ClearContentsDuringUpdate="false"
TargetControlID="Label1" ServiceMethod="GetSlowThing" ServicePath="~/Services/WebService1.asmx" />
</p>
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
DataKeyNames="Id" ItemType="WebApplication2015.DAL.Table1" Width="100%" SelectMethod="GridView1_GetDataAsync"
AllowPaging="True" PageSize="5" AllowSorting="True">
<Columns>
<asp:BoundField DataField="Id" HeaderText="Id" SortExpression="Id" />
<asp:BoundField DataField="FieldOne" HeaderText="Field One" SortExpression="FieldOne" />
<asp:BoundField DataField="FieldTwo" HeaderText="Field Two" SortExpression="FieldTwo" />
<asp:TemplateField HeaderText="Field Three">
<ItemTemplate>
<asp:TextBox ID="TB1" runat="server" Text="Empty"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</asp:Content>

Comments: I was able to make it work provided I used the PopulateTriggerControlID. Without this, the web service did not get called. That is the crux of it. Since it works when I click the populate control, I assume my web service works OK. It is one of the old style SOAP web services because that is what I've seen in the examples. Can this be used with a .svc service? Basically then, how do you kick of the web service query when the page loads? Thanks