AJAX :: AutoCompleteExtender Control Not Working

I tested the web service.it is working fine.The problem that i want to solve is to make call to webservice. Here is the .aspx code

<asp:TextBox ID="txtFullName" runat="server"></asp:TextBox>
<ajaxToolkit:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"
TargetControlID="txtFullName" ServiceMethod="FindFullName"
ServicePath="../WebService.asmx" EnableCaching="true"></ajaxToolkit:AutoCompleteExtender>
Webservice code :
using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Configuration;
using System.Data.SqlClient;
using System.Data;
using System.Collections;
using System.Collections.Generic;
/// <summary>
/// Summary description for WebService
/// </summary>
[WebService(Namespace = "[URL]/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class WebService : System.Web.Services.WebService {
public WebService () {
//Uncomment the following line if using designed components
//InitializeComponent();
}
[WebMethod]
public string HelloWorld() {
return "Hello World";
}
[WebMethod]
public string[] FindFullName(string prefixText)
{
string sql = "Select FName from Investigators Where FName like @prefixText";
SqlDataAdapter da = new SqlDataAdapter(sql,"Data Source=vvvvv;Initial Catalog=vyyyd;Persist Security Info=True;User ID=PowetttrUsyyer;Password=888888);
da.SelectCommand.Parameters.Add("@prefixText", SqlDbType.VarChar, 50).Value = prefixText+ "%";
DataTable dt = new DataTable();
da.Fill(dt);
List<string> liste = new List<string>();
foreach (DataRow dr in dt.Rows)
{
liste.Add(dr.ItemArray[0].ToString());
}
return liste.ToArray();
}
}