Created 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>