Problem in (mvc 3 + razor ) application

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C#

MVC

My application developed in mvc 3 + razor , I have two page add detail and edit detail , and a button named Estimation on both page , When i click on the button i open a model jquery popup of Estimation form.
Estimation page having a dropdown of year's when i select year i populate all rates by following script.

<script type ="text/javascript">

    $(function () {
        $('#RatePeriodid').change(function () {
            var selectedValue = $(this).val();
            $.ajax({
                type: "POST",
                url: '../WebServices/AddSOP.asmx/PopulateEstimationRates',
                data: "{Period_id:'" + selectedValue + "'}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                async: false,
                success: function (msg) {
                   
                    var data = msg.d;
                    $('#Standerd_Weekday_Rate').val(data.Standerd_Weekday_Rate);
                     .................................
		     and so on 
                   
                },
                error: function (msg) {
                    alert("Error");


                }
            });

        });
    });
     
</script> 

My problem is when i open popup from add detail page all things are working fine , but when i open popup from edit detail page , and select an year from dropdown my ajex script giving error message. And my web service (../WebServices/AddSOP.asmx/PopulateEstimationRates) not calling .
What is the wrong with my code ?