auto complete is not firing

<asp:TextBox ID="txtRegistrationNo" runat="server" MaxLength="13" CssClass="textfield" AutoPostBack="true">
                                                    <asp:AutoCompleteExtender ID="TextBox1_AutoCompleteExtender" runat="server" DelimiterCharacters="" Enabled="True" ServiceMethod="GetAutoCompleteData" MinimumPrefixLength="1" EnableCaching="true" TargetControlID="txtRegistrationNo">

[System.Web.Script.Services.ScriptMethod()]
[System.Web.Services.WebMethod]
public static List<string> GetAutoCompleteData(string prefixname)
{
    List<string> result = new List<string>();
    List<e_location_master> locationmaster = (List<e_location_master>)locationlist;
    locationmaster = locationmaster.Where(P => P.STATE.ToLower().Contains(prefixname.ToLower()) || P.STATE.ToLower().Contains(prefixname.ToLower())).ToList();
    result = locationmaster.Select(P => P.STATE).Take(10).ToList();
    
    return result;
}