Textboxwatermark not working

Question

i am having a textbox and a button. i want to apply textbox watermark to this textbox.

i have registerd ajax toolkit

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

and i have all ajax tools in my toolbox . i have a script manager .

    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>

i dragged a textboxwaermark on form .

<asp:TextBoxWatermarkExtender ID="TBWE2" runat="server"
    TargetControlID="TextBox2"
    WatermarkText="Type First Name Here"
    WatermarkCssClass="watermarked" />


till now every thing seems ok

but this tbwatermark not working . there is  no error, no exception.
 then why ?????

some one suggest pls . i am finding it for last 2 days.
thanks in advance