SliderExtender not displaying

I have the following statements in the master page of my site default page:

   <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="act" %>
   <act:ToolkitScriptManager ID="tsmLibrary" runat="server" />

I have the following statements in a user control that is included in the default page:

   <%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="act" %>
   <asp:TextBox ID="txtChapter3" runat="server" />    
   <act:SliderExtender ID="seChapter3" runat="server" Length="400" Maximum="100" Minimum="1" TargetControlID="txtChapter3" />

The text box is displaying as a normal rectangular text box, not as a slider. I've put the same code on a blank new page and it displays fine. I cannot figure out why it's not displaying as a slider in the user control. Any ideas? Thanks!