AJAX AutoComplete not firing

In my code I have an AJAX AutoComplete, looks like this:

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><ajaxToolkit:AutoCompleteExtender ID="TextBox1_AutoCompleteExtender" runat="server" DelimiterCharacters="" Enabled="True" ServicePath="AutoComplete.asmx" ServiceMethod="GetCompletionList" TargetControlID="TextBox1" CompletionSetCount="10" MinimumPrefixLength="2"></ajaxToolkit:AutoCompleteExtender>

Its fairly straight forward and I have done this kind of thing connecting the Web Service from SQL, however this time within the Web Service I wont to connect to a array from strings. The strings will be the network ID's. The web service looks like:

Imports System.DirectoryServices
Imports System.DirectoryServices.AccountManagement
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel

' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.

Public Class WebService2
    Inherits System.Web.Services.WebService

    <WebMethod()> _
    Public Function GetCompletionList(ByVal prefixText As String, ByVal count As Integer) As List(Of String)

        Dim ctx As New PrincipalContext(ContextType.Domain)

        Dim group As GroupPrincipal = GroupPrincipal.FindByIdentity(ctx, "####")
        ' Create a list of strings.
        Dim list As New List(Of String)
        ' if found....
        If group IsNot Nothing Then
            ' iterate over members            
            For Each p As Principal In group.GetMembers()
                list.Add(p.SamAccountName)
            Next
        End If

        list.Sort()

        Return list
    End Function

I could chuck the lot into a database beforehand but wouldn't mind trying to solve the problem in the first instance. Anyone got any suggestions?...

:)