Asp.net Calculator answer textbox field will not change when adding new numbers, Why?

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ASP.NET

Hello. I have made a calculator for five textboxes. The first two will subtract two numbers and the answer will be in the third textbox. The other three textboxes are adding three numbers and the answer is in the fourth textbox. I also have the textboxes with the answer in them to be greyed out. What is happening is that the answer fields can not be changed. If you put in the calculations and click the calculate button it will give you the answer but if you change the numbers and hit calculate button the answer will stay the same. What did I do wrong or what am I missing?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Drawing;
using System.Text;



public partial class FinancialIndicatorsFormA : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void ButtonCalculate_Click(object sender, EventArgs e)
    {
        int a = Convert.ToInt32(TextBoxTA.Text);
        int b = Convert.ToInt32(TextBoxTL.Text);
        TextBoxTNA.Text = Convert.ToString(a - b); 

    }
    protected void ButtonCalculate2_Click(object sender, EventArgs e)
    {
        int a = Convert.ToInt32(TextBoxNPRNA.Text);
        int b = Convert.ToInt32(TextBoxETRNA.Text);
        int c = Convert.ToInt32(TextBoxTUNA.Text);
        TextBoxTNA2.Text = Convert.ToString(a + b + c); 
    }
}