Bug: Program doesn't recognise uppercase letters in guesses, even with toLowerCase()

public class Game {
  //variable declared but not initialised
  private String mAnswer;
  public static final int MAX_MISSES = 7; 

  //initialise hit and miss guess variables. 
  private String mHits;
  private String mMisses;

  //constructor function that takes an answer in form of a string
  public Game(String answer) {
     mAnswer = answer;   
       mHits = "";
     mMisses = "";  
  }

  private char validateGuess(char letter){
   if(! Character.isLetter(letter)) {
      throw new IllegalArgumentException("A letter is required");
   }
   letter = Character.toLowerCase(letter);
   if(mMisses.indexOf(letter) >= 0 || mHits.indexOf(letter) >= 0) {
       throw new IllegalArgumentException("Sorry, " + letter + " has already been guessed. Try another letter"); 
   }
   return letter;
}


  public boolean applyGuess(String letters) {
    if(letters.length() == 0) {
      throw new IllegalArgumentException("Error: We found no letter. Please type a letter");
    }
     char firstLetter = letters.charAt(0);
     return applyGuess(firstLetter);
    }


  //add letters to hit and misses depending on guess
  public boolean applyGuess(char letter) {
     letter = validateGuess(letter);
     boolean isHit = mAnswer.indexOf(letter) >= 0;
     if (isHit) {
        mHits += letter;
        return true;

     }  else {
        mMisses += letter;
        return false;

     }
  }

  //method to display the current progress of the game 
  public String getCurrentProgress() {
    //empty string
    String progress = "";
    //loop through each letter of the answer   
    for (char letter : mAnswer.toCharArray()) {
    //default word display that hides the letter
    char display = '-';

    //if the letter is guessed correctly add that letter to progress
    if(mHits.indexOf(letter) >= 0) {
       //change display to the letter
       display = letter;
    }

    progress += display;

  }
return progress;
}

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

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

public String getAnswer() {
  return mAnswer;
}


}

//Import package
import java.io.Console;

public class Prompter {

  //private member variable
  private Game mGame;   

  //constructor function
  public Prompter (Game game) { 
      //set member variable equal to what is passed in
      mGame = game;
   }

  public void playTheGame() {
    while(mGame.getRemainingTries() > 0 && !mGame.isSolved()) {
     displayProgress();
     promptForGuess();
    }

    if(mGame.isSolved()) {
      System.out.printf("You won the game with %d tries remaining.\n", mGame.getRemainingTries());
    } else {    
      System.out.printf("Unlucky! The word was %s.  :(\n", mGame.getAnswer());
    }
  }

  //a prompter method for the user to guess new letters  
  public boolean promptForGuess() {

      //store console Object and set up prompter
      Console console = System.console();

      boolean isHit = false;
      boolean isValidGuess = false;

      //while the guess is not valid catch the iae 
      while(! isValidGuess) {
        String guessAsString = console.readLine("Enter a letter: ");

        try {
          isHit = mGame.applyGuess(guessAsString);
          isValidGuess = true;
        } catch (IllegalArgumentException iae) {
         console.printf("%s.  Please try again \n", iae.getMessage());
        }
      }

      //define new guess in a char variable.
      return isHit;

  } 


  public void displayProgress() {
      System.out.printf("You have %d guesses remaining to solve:  %s.\n", 
                          mGame.getRemainingTries(),
                          mGame.getCurrentProgress());
  }


}

public class Hangman {

    public static void main(String[] args) {
        // Enter amazing code here:

        if(args.length == 0) {

             System.out.println("Please enter a word");
             System.exit(0);

           }

           //enter the first element of args array as argument
           Game game = new Game(args[0]);


        //Supply a string to start the game.
        //Game game = new Game("antidisestablishmentarism");

        //The following code calls the prompterForGuess method with a new Prompter object.
        Prompter prompter = new Prompter(game);

        prompter.playTheGame();
        //variable from Prompter object assigned to method.
       /* prompter.displayProgress();
        boolean isHit = prompter.promptForGuess();

        if(isHit) {
             System.out.println("We got a Hit!");
        } else {
             System.out.println("Whoops that was a miss");  
        }
        prompter.displayProgress();*/
    }

}