Knockout js grid ajax call not working

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Knockout.js

grid have price, qty column. i need to calculate the total ie. price * qty. Its working new row only. but am fetching the data from web service and loaded it into the grid. now am chaning the qty value but the calculation is not working. its working adding a new row. Please help..

What I have tried:

<script>

        var quotationViewModel = function (quotations) {
            var self = this;

            self.quotations = ko.observableArray(ko.utils.arrayMap(quotations, function (item) {
                return new Item(item);
            }));

            self.addquotation = function () {
                self.quotations.push(new Item());

                UIValidation();
            };

            self.removequotation = function (quotation) {
                self.quotations.remove(quotation);
            };
            self.savequotation = function () {
                return ko.utils.stringifyJson(self.quotations);
            }
            self.grand_total = ko.computed(function () {
                var total = 0;
                for (var p = 0; p < self.quotations().length; ++p) {
                    total += self.quotations()[p].total();
                }
                return total.toFixed(2);
            });

            self.Tax_Total = ko.computed(function () {
                var tax = 0;
                for (var p = 0; p < self.quotations().length; ++p) {
                    tax += self.quotations()[p].tax_calc();
                }
                return tax.toFixed(2);
            });

            self.grandTax = ko.computed(function () {
                var tax = 0, sum = 0, totalsum = 0;
                for (var p = 0; p < self.quotations().length; ++p) {
                    tax += self.quotations()[p].tax_calc();
                    sum += self.quotations()[p].total();
                    totalsum = tax + sum;
                }
                $('#grandTax').val(totalsum.toFixed(2));
                $('#totalAmt').val(totalsum.toFixed(2));
                $('#TotalVat_Amt').val(tax.toFixed(2));
                return totalsum.toFixed(2);
            });

            function Item(item) {
                var self = this;
                self.P055_ID = ko.observable('');
                self.itemCode = ko.observable('');
                self.itemDesc = ko.observable('');
                self.packing = ko.observable();
                self.VatCode = ko.observable();
                self.VatDesc = ko.observable();
                self.taxID = ko.observable();
                self.tax_perc = ko.observable(0);
                self.quantity = ko.observable(0);
                self.rate = ko.observable(0);
                self.remarks = ko.observable();

                for (var k in item)
                    self[k] = ko.observable(item[k]);

                self.total = ko.dependentObservable(function () {
                    return ((self.quantity() * self.rate()));

                }).bind(self);

                self.tax_calc = ko.dependentObservable(function () {
                    return ((self.tax_perc() * self.total()) / 100);

                }).bind(self);
                self.Tax_Total = ko.observable();

            }
        };

        var quotationModel = new quotationViewModel(@Html.Raw(@Model.gridQuotationDetails));
        ko.applyBindings(quotationModel, document.getElementById('divQuotationDetails'));

    </script>