Why isn't my code working? It keeps showing "enter a letter: "

Hangman.java
public class Hangman{

  public static void main(String[] args){

    Game game = new Game("treehouse");
    Prompter prompter = new Prompter(game);
    while (game.getRemainingTries() > 0 && !game.isWon()){
      prompter.promptForGuess();
      prompter.displayProgress();
    }
    prompter.displayOutcome();
  }
}

Game.java
class Game{
  public static final int MAX_MISSES = 7;
  private String answer;
  private String hits;
  private String misses;

  public String getAnswer(){
    return answer;
  }

  public Game(String answer){
    this.answer = answer.toLowerCase();
    hits = "";
    misses = "";  
  }

  private char normalizeGuess(char letter){
    if(! Character.isLetter(letter)){
      throw new IllegalArgumentException("A letter is required");
    }
    letter = Character.toLowerCase(letter);
    if(misses.indexOf(letter) != -1 || hits.indexOf(letter) != -1){
      throw new IllegalArgumentException(letter+"has already been guesses");
    }
    return letter;
  }

    public boolean applyGuess(String letters){
      if(letters.length() == 0){
        throw new IllegalArgumentException("No letter found");
      }
      return applyGuess(letters.charAt(0));
    }

  public boolean applyGuess(char letter){
    letter = normalizeGuess(letter);
    boolean isHit = answer.indexOf(letter) != -1;
    if (isHit){
      hits += letter;
    } else{
      misses += letter; 
    }
    return isHit;
  }

  public int getRemainingTries() {
    return MAX_MISSES - misses.length();
  }

  public String getCurrentProgress() {
    String progress = "";
    for(char letter : answer.toCharArray()) {
      char display = '-';
      if(hits.indexOf(letter) != -1){
        display = letter;
      }
      progress += display;
    }
    return progress;
  }

  public boolean isWon(){
    return getCurrentProgress().indexOf('-') == -1;
  }
}

prompter.java
import java.util.Scanner;

class Prompter{
  private Game game;

  public Prompter(Game game){
  this.game = game;
  }

  public boolean promptForGuess(){
    Scanner scanner = new Scanner(System.in);
    boolean isHit = false;
    boolean isAcceptable = false;

    do{
        System.out.print("Enter a letter:   ");
        String guessInput = scanner.nextLine();

        try{
          isHit = game.applyGuess(guessInput);
          }catch(IllegalArgumentException iae){
          System.out.printf("%s. Please try again. %n",iae.getMessage());
        }
      }while(! isAcceptable);
        return isHit;
    }

  public void displayProgress(){
    System.out.printf("You have %d tries left to solve:   %s%n",
                      game.getRemainingTries(),
                      game.getCurrentProgress()); 
  }

  public void displayOutcome(){
    if(game.isWon()){
    System.out.printf("You won with %d tries remaining!%n",game.getRemainingTries());
    }else{
      System.out.printf("Bummer! The word was %s.%n",game.getAnswer());
  }
}
}