AutoComplete TextBox using Ajax controls

I have a textbox i.e. myTextBox which and use the ajaxtoolkit autocompleteextender as follow:

ajaxToolkit:AutoCompleteExtender runat="server" ID="autoComplete1" TargetControlID="myTextBox" ServiceMethod="GetCompletionList" MinimumPrefixLength="2" </ajaxToolkit:AutoCompleteExtender>

Code Behind is as follows

[System.Web.Services.WebMethod] [System.Web.Script.Services.ScriptMethod] public string[] GetCompletionList(string prefixText, int count) {

I have my code here

}

My problem is that it Never calls the GetCompletionList - the web service method is never called. It never enters the public string GetCompletionList I really do not know why! Do you have any ideas? Thanks