Blank panel - When loading panel from another class (frame created in the base class)

I have created 3 classes (code given below).

BasicDetails - base class - frame created here along with a panel GradeSubjectList - derived class - another panel created here and loaded in the frame created in the base class SubjectList - derived class - another panel created here and loaded in the frame created in the base class

Base class has a panel of its own and on click of the button loads the panel created in GradeSubjectList. A button created in this panel calls a function in base class which inturn loads the panel created in SubjectList in the frame created in the base class.

The panel created in BasicDetails and GradeSubjectList load with any issues, and when the panel created in SubjectList is being loaded in the frame created in the base class displays only in a blank frame

public class BasicDetails{

    static int lec_no;
    static int sub_no;

    JFrame frame = new JFrame("Time Table");
    Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    JPanel panel_bd = new JPanel();

    public static void main(String[] args) {
        BasicDetails bd = new BasicDetails();
        bd.main_method_bd();
    }

    public void main_method_bd() {

        panel_bd.setBackground(Color.white);
        frame.setExtendedState(java.awt.Frame.MAXIMIZED_BOTH);
        panel_bd.setLayout(null);

        JLabel lec_no_lbl = new JLabel("Enter the number of lectures required per day:");
        JTextField lec_no_txt = new JTextField(5);
        lec_no_txt.setText("5");

        JButton next_btn_1 = new JButton("Next");

        panel_bd.add(lec_no_lbl);
        panel_bd.add(lec_no_txt);
        panel_bd.add(next_btn_1);

        lec_no_lbl.setBounds(5,100,300,20);
        lec_no_txt.setBounds(320,100,50,20);
        next_btn_1.setBounds(520,350,100,20);

        next_btn_1.addActionListener(e->
        {

            lec_no=Integer.parseInt(lec_no_txt.getText());
            panel_bd.setVisible(false);
            gsl(); //calls function to load the next panel
        });

        frame.setSize(screenSize.width,screenSize.height);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        frame.add(panel_bd);        
    }

    //----this function loads the panel from GradeSubjectList.java
    public void gsl() {

            GradeSubjectList GSL = new GradeSubjectList();
            JPanel gsl_panel = GSL.main_method_gsl();
            frame.add(gsl_panel);   
    }

    //-----this function loads the panel from SubjectList.java (this function gets called from GradeSubjectList.java but the panel does not load)
    public void sl() {

        SubjectList sl = new SubjectList();
        JPanel sl_panel = sl.main_mehod_sl();
        frame.add(sl_panel);        
    }   
}

public class GradeSubjectList extends BasicDetails{

    JComboBox grd_cb = new JComboBox();
    JComboBox sub_cb_1 = new JComboBox();

    public JPanel main_method_gsl() {

        //-------Panel creation and loading components------------
        JPanel panel_gsl = new JPanel();
        panel_gsl.setBackground(Color.blue);
        panel_gsl.setLayout(null);

        JLabel tec_details_txt = new JLabel("Select the Grades and Subjects");
        JButton next_btn_5 = new JButton("Next");

        next_btn_5.setBounds(600,500,150,20);
        tec_details_txt.setBounds(100,50,250,20);

        grd_cb.setBounds(10,100,100,20);
        sub_cb_1.setBounds(130,100,100,20);

        panel_gsl.add(tec_details_txt);
        panel_gsl.add(next_btn_5);
        panel_gsl.add(grd_cb);
        panel_gsl.add(sub_cb_1);

        //On click perform actions
        next_btn_5.addActionListener(e->
        {
            panel_gsl.setVisible(false);                        
            sl(); //---------calls function in BasicDetails.java to load the next panel
        });

        return(panel_gsl); ////------returns panel to BasicDetails.java to load it in the frame
    }
}

public class SubjectList extends BasicDetails{

    public JPanel main_mehod_sl() {
        //-------Panel creation and loading components------------
        
        JPanel panel_sl = new JPanel();
        panel_sl.setBackground(Color.white);
        panel_sl.setLayout(null);

        JLabel sub_list_lbl = new JLabel("Enter Subject Name");
        JLabel max_day_lbl = new JLabel("Max Lectures Per Day");
        JLabel max_week_lbl = new JLabel("Max Lectures Per Week");

        sub_list_lbl.setBounds(25,50,150,20);
        max_day_lbl.setBounds(200,50,150,20);
        max_week_lbl.setBounds(375,50,150,20);

        JButton next_btn_2 = new JButton("Next");
        next_btn_2.setBounds(600,500,150,20);

        JTextField sub_name_txt = new JTextField();
        JTextField max_day_txt = new JTextField();
        JTextField max_week_txt = new JTextField();
        sub_name_txt.setBounds(25,100,150,20);
        max_day_txt.setBounds(200,100,105,20);
        max_week_txt.setBounds(375,100,150,20);

        panel_sl.add(sub_list_lbl);
        panel_sl.add(max_day_lbl);
        panel_sl.add(max_week_lbl);
        panel_sl.add(next_btn_2);
        panel_sl.add(sub_name_txt);
        panel_sl.add(max_day_txt);
        panel_sl.add(max_week_txt);
        
        //On click action
        next_btn_2.addActionListener(e->
        {
            panel_sl.setVisible(false);         
        });

        return(panel_sl); //------returns panel to BasicDetails.java to load it in the frame
    }
}

I have changed the order in which the panels are loaded from other classes. I noticed that the panel which is being loaded first (from the other class) does load without any problem while the other panel does not.

for e.g. if the order is

1. GradeSubjectList - panel loads properly
2. SubjectList - panel does not show

and is the order is changed

1. SubjectList - panel loads properly
2. GradeSubjectList - panel does not show

So my guess is the most of the code is working fine...cannot just figure out where it is going wrong...