Strange problems with browser rendering javascript [VS 2010]

[venerable bede]

I have the following just before my </body>

 <script type="text/javascript">
           //success message..
           function OnSuccess(result) {
               if (result) {
                   document.getElementById("<%= lblMessage.ClientID %>").textContent = result;

               }
           }
function CheckForReturnAndComplete() {
               if (event.keyCode === 13) {
                   var txt1 = document.getElementById('<%=text1.ClientID %>').value;
                   var txt2 = document.getElementById('<%=text2.ClientID %>').value;
                   PageMethods.ScanComplete(txt1, txt2, OnSuccess, OnFailure);

               }
           }
 </script>

My WebMethod in the code behind (cut down for simplicity) :

  [WebMethod]
        public static string ScanComplete(string data1,string data2)
        {
            string returnValue = string.Empty;
            returnValue = data1 + " and " + data2 + " have been saved.";
            return returnValue;
        }

Basically when the user hits carriage return on the second textbox 'text2' the contents of text1 and text2 are sent to the [WebMethod], concatenated and returned to be displayed in a label.

This works great on my dev machine but when I try it from any another machine the label does not appear. It's been a while since I have done any web work so am a wee bit confused.

Anyone have any ideas why this is happening?

Thanks in Advance