AjaxManager not updating!

[Coax]

I have a RadAjaxManager, which should update a label on click of a button (simplest of tasks I assume.)

<telerik:RadAjaxLoadingPanel ID="AjaxLoadingPanel1" runat="server" Height="75px" Width="75px">

<asp:Label runat="server" ID="lblTest" Text="appears here"/><br/>

<asp:Button runat="server" OnClick="btnTest_OnClick" ID="btnTest" Text="Test"/>

        <telerik:RadAjaxManager runat="server" ID="AjaxManager1">
            <AjaxSettings>
                <telerik:AjaxSetting AjaxControlID="btnTest">
                    <UpdatedControls>
                        <telerik:AjaxUpdatedControl ControlID="lblTest" LoadingPanelID="AjaxLoadingPanel1"/>
                    </UpdatedControls>
                </telerik:AjaxSetting>
            </AjaxSettings>
        </telerik:RadAjaxManager>

and in the code behing

 

        protected void btnTest_OnClick(object sender, EventArgs e)
        {
            lblTest.Text = "12324";
        }

No matther how many times I try, the label could not get updated.
I've used fiddler and part of the response is something like below:

31|updatePanel|lblTestPanel|<span id="lblTest">12324</span>|53|updatePanel|AjaxManager1SU|<span id="AjaxManager1" style="display:none;"></span>

Am I missing anything?

[Coax]

I could spot the problem.
I have used Response.Write() in the page, and it's not supported!