Implementation of Navision forms in .NET Web Application

Hello,

In my testcases, UPDATE function of WEB SERVICE is not doing the Validation of other fields done from Coding part.
e.g.

private void textBox1_TextChanged(object sender, EventArgs e)
        {
            Sales_Order_Service ss1 = new Sales_Order_Service();
            Sales_Order so1 = new Sales_Order();
            ss1.Url = "http://isplserver:7047/DynamicsNAV/WS/CRONUS_India_Ltd/Page/Sales_Order";

            ss1.UseDefaultCredentials = true;
            so1.No = textBox1.Text;
            so1 = ss1.Read("Order", so1.No);
            textBox3.Text = so1.Sell_to_Customer_No;
            textBox2.Text = so1.Sell_to_Post_Code;
            textBox4.Text = so1.Sell_to_City;            
        }

        private void textBox2_TextChanged(object sender, EventArgs e)
        {
            Sales_Order_Service ss1 = new Sales_Order_Service();
            Sales_Order so1 = new Sales_Order();
            ss1.Url = "http://isplserver:7047/DynamicsNAV/WS/CRONUS_India_Ltd/Page/Sales_Order";

            ss1.UseDefaultCredentials = true;
            so1.No = textBox1.Text;
            so1 = ss1.Read("Order", so1.No);
       
            so1.Sell_to_Post_Code = textBox2.Text;
            ss1.Update(ref so1);
            textBox3.Text = so1.Sell_to_Customer_No;
            textBox2.Text = so1.Sell_to_Post_Code;
            textBox4.Text = so1.Sell_to_City;            
        
        }

Then If i go to update Sell-To Post Code field of Sales Header using Sales Order web service then logically it should update the Sell-To City field of the same record


But, It only updates the Post Code and leave City as it is.

So I want to make confirm whether I have done any mistake in my code or it's a bufg in Update Function of WEB SERVICE.

Please, help me urgently.
Thanks in advance.

could share what was the problem and solution?