Ajax Progressmeter not showing

I am trying to display an ajax progess meter on a webpage, but nothing happens.

The code:

<asp:UpdatePanel ID="updPMain" runat="server" UpdateMode="conditional"><ContentTemplate><asp:Label ID="lblCriteria" runat="server" Text=""></asp:Label><br /><asp:ListView ID="lstResults" runat="server" RenderOuterTable="false"><ItemTemplate><div><h3><%# DataBinder.Eval(Container.DataItem, "Name")%></h3></div><table border="1"><tr><td><asp:Image ID="imgMain" runat="server" ImageUrl=<%#DataBinder.Eval(Container.DataItem, "Image")%>></asp:Image></td><td rowspan="2"><%# DataBinder.Eval(Container.DataItem, "Description")%></td></tr><tr><td><%# DataBinder.Eval(Container.DataItem, "City")%> </td></tr></table><asp:Panel ID="pHeader" runat="server" BackColor="#CCCCCC"><table class="table1" ><tr><td><asp:Image ID="Image2" runat="server" ImageUrl="~/images/expand.jpg"/></td><td>In de buurt...</td><td><asp:Label ID="lblShowHide" runat="server" ></asp:Label></td></tr></table></asp:Panel><br /><asp:Panel ID="pnlChildren" runat="server"><asp:Label ID="Label1" runat="server" Text="test"></asp:Label></asp:Panel><asp:CollapsiblePanelExtender ID="pnlChildren_CollapsiblePanelExtender"
                                                        runat="server"
                                                        Enabled="True"
                                                        TargetControlID="pnlChildren"
                                                        CollapsedSize="0"
                                                        ExpandedSize="200"
                                                        Collapsed="True"
                                                        ExpandControlID="pHeader"
                                                        CollapseControlID="pHeader"
                                                        AutoCollapse="False"
                                                        AutoExpand="False"
                                                        ScrollContents="True"
                                                        TextLabelID="Label1"
                                                        CollapsedText=""
                                                        ExpandedText=""
                                                        ExpandDirection="Vertical"
                                                        ImageControlID="Image1"
                                                        ExpandedImage="~/images/collapse.jpg"
                                                        CollapsedImage="~/images/expand.jpg"></asp:CollapsiblePanelExtender><hr /></ItemTemplate></asp:ListView><br /></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="txtSliderBuffer" EventName="TextChanged" /></Triggers></asp:UpdatePanel><asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="updPMain" DisplayAfter="0" DynamicLayout="true"><ProgressTemplate>
          Update in progress...</ProgressTemplate></asp:UpdateProgress>

It seems that the update panel takes much more time to complete since I added the progress meter, but absolutely nothing shows up ...

Thanks,

Tom