Why launching does it random?

When i run this program sometimes shows me all buttons, but sometimes only 2 or 3 or 4 or 5 or even just 1.. why is that?? I really do not get it. There should always be 6 buttons, but it doesnt show them. Is there any logical reason?

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class testnet
{
        public static void main (String[] args)
        {
                JFrame frame = new JFrame("Knjigarna");
                frame.setVisible(true);
                frame.setSize(800,200);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                JPanel panel = new JPanel();
                frame.add(panel);
                
                JButton button1 = new JButton("Prikazi vse");
                panel.add(button1);
                button1.addActionListener (new Action1());

                JButton button2 = new JButton("Prikazi knjigo");
                panel.add(button2);
                button2.addActionListener (new Action2());
                
                JButton button3 = new JButton("Dodaj knjigo");
                panel.add(button3);
                button3.addActionListener (new Action3());
                
                JButton button4 = new JButton("Brisi knjigo");
                panel.add(button4);
                button4.addActionListener (new Action4());
                
                JButton button5 = new JButton("Uredi knjigo");
                panel.add(button5);
                button5.addActionListener (new Action5());
                
                JButton button6 = new JButton("Izhod");
                panel.add(button6);
                button6.addActionListener (new Action6());
        }
        
        static class Action1 implements ActionListener
        {
                public void actionPerformed (ActionEvent e) 
                {     
                        JFrame frame2 = new JFrame("Pikaz vseh knjig");
                        frame2.setVisible(true);
                        frame2.setSize(500,800);
                        JLabel label = new JLabel("Seznam vseh knjig:");
                        JPanel panel = new JPanel();
                        JTextField text1=new JTextField("Naslov: ");
                        JTextField text2=new JTextField("Avtor: ");
                        frame2.add(panel);
                        panel.add(label);  
                        panel.add(text1);
                        panel.add(text2);
                }
        }
        
        static class Action2 implements ActionListener
        {        
                public void actionPerformed (ActionEvent e) 
                {     
                        JFrame frame3 = new JFrame("Prikaz knjige");
                        frame3.setVisible(true);
                        frame3.setSize(600,300);

                        JLabel label = new JLabel("Vpisi naslov knjige:");
                        JPanel panel = new JPanel();
                        frame3.add(panel);
                        panel.add(label);
                }
        }
        static class Action3 implements ActionListener
        {        
                public void actionPerformed (ActionEvent e) 
                {     
                        JFrame frame4 = new JFrame("Dodajanje knjige");
                        frame4.setVisible(true);
                        frame4.setSize(600,300);

                        JLabel label = new JLabel("Vpisi podtke o knjigi");
                        JPanel panel = new JPanel();
                        frame4.add(panel);
                        panel.add(label);
                }
        }   
        
        static class Action4 implements ActionListener
        {        
                public void actionPerformed (ActionEvent e) 
                {     
                        JFrame frame5 = new JFrame("Brisanje knjige");
                        frame5.setVisible(true);
                        frame5.setSize(600,300);

                        JLabel label = new JLabel("Vpisi naslov knjige, ki jo zelis brisati");
                        JPanel panel = new JPanel();
                        frame5.add(panel);
                        panel.add(label);
                }
        }   
        
        static class Action5 implements ActionListener
        {        
                public void actionPerformed (ActionEvent e) 
                {     
                        JFrame frame6 = new JFrame("Urejanje knjige");
                        frame6.setVisible(true);
                        frame6.setSize(600,300);

                        JLabel label = new JLabel("Vpisi naslov knjige, ki jo zelis urejati");
                        JPanel panel = new JPanel();
                        frame6.add(panel);
                        panel.add(label);
                }
        }   
        
        static class Action6 implements ActionListener
        {        
                public void actionPerformed (ActionEvent e) 
                {     
                        System.exit(0);
                }
        }   
}