Spring Boot app randomizer stops before it runs through the entire array

I have a simple quiz app in Spring Boot, that should show random questions (20 questions in total.) But the app instead stops at a random number of questions, sometimes 4/20, sometimes 15/20, etc. Not sure where the mistake is. Here's my code:

public class Quiz implements ActionListener {

List<Integer> questionIndexes = new ArrayList<>();
String[] questions = {
        // 20 different questions
};
String[][] options = {
        // Four options per question
};
char[] answers =  {
        // Answers
};

char answer;
int index;
int correctGuesses = 0;
int totalQuestions = questions.length;
int result;
int seconds = 10;

Timer timer = new Timer(1000, e -> {
    seconds--;
    secondsLeft.setText(String.valueOf(seconds));
    if (seconds <= 0) {
        displayAnswer();
    }
});

public Quiz() {

    for (int i = 0; i < totalQuestions; i++) {
        questionIndexes.add(i);
    }
    Random random = new Random();
    index = random.nextInt(totalQuestions);
    answer = answers[index];

    jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    // and the rest of the code for UI

    nextQuestion();
}

public void nextQuestion() {
    if (index >= totalQuestions) {
        results();
    } else {
        index++;
        textField.setText("Question " + (index + 1));
        textArea.setText(questions[index]);
        answerLabelA.setText(options[index][0]);
        answerLabelB.setText(options[index][1]);
        answerLabelC.setText(options[index][2]);
        answerLabelD.setText(options[index][3]);
        timer.start();
    }
}

@Override
public void actionPerformed(ActionEvent e) {
    buttonA.setEnabled(false);
    buttonB.setEnabled(false);
    buttonC.setEnabled(false);
    buttonD.setEnabled(false);

    if(e.getSource() == buttonA) {
        answer = 'A';
        if(answer == answers[index]) {
            correctGuesses++;
        }
    }
    if(e.getSource() == buttonB) {
        answer = 'B';
        if(answer == answers[index]) {
            correctGuesses++;
        }
    }
    if(e.getSource() == buttonC) {
        answer = 'C';
        if(answer == answers[index]) {
            correctGuesses++;
        }
    }
    if(e.getSource() == buttonD) {
        answer = 'D';
        if(answer == answers[index]) {
            correctGuesses++;
        }
    }
    displayAnswer();
}

public void displayAnswer() {

    timer.stop();

    buttonA.setEnabled(false);
    buttonB.setEnabled(false);
    buttonC.setEnabled(false);
    buttonD.setEnabled(false);

    if(answers[index] != 'A') {
        answerLabelA.setForeground(new Color(255,0,0));
    }
    if(answers[index] != 'B') {
        answerLabelB.setForeground(new Color(255,0,0));
    }
    if(answers[index] != 'C') {
        answerLabelC.setForeground(new Color(255,0,0));
    }
    if(answers[index] != 'D') {
        answerLabelD.setForeground(new Color(255,0,0));
    }

    Timer pause = new Timer(2000, e -> {
        answerLabelA.setForeground(new Color(210,210,210));
        answerLabelB.setForeground(new Color(210,210,210));
        answerLabelC.setForeground(new Color(210,210,210));
        answerLabelD.setForeground(new Color(210,210,210));

        answer = ' ';
        seconds = 10;
        secondsLeft.setText(String.valueOf(seconds));
        buttonA.setEnabled(true);
        buttonB.setEnabled(true);
        buttonC.setEnabled(true);
        buttonD.setEnabled(true);
        questionIndexes.remove(Integer.valueOf(index));
        index++;
        nextQuestion();
    });
    pause.setRepeats(false);
    pause.start();
}

public void results() {
    buttonA.setEnabled(false);
    buttonB.setEnabled(false);
    buttonC.setEnabled(false);
    buttonD.setEnabled(false);

    result = (int)((correctGuesses/(double)totalQuestions) * 100);

    textField.setText("Your score: ");
    textArea.setText("");
    answerLabelA.setText("");
    answerLabelB.setText("");
    answerLabelC.setText("");
    answerLabelD.setText("");

    numberRight.setText("(" + correctGuesses + "/" + totalQuestions + ")");
    percentage.setText(result + "%");
    jFrame.add(numberRight);
    jFrame.add(percentage);
}

}

And every time I run the code, it randomly shows 5, 18, or 9 questions, and then the app stops, instead of running through all 20 questions randomly.