privateの変数、ｘとｙが意図したとおりに変化できない

このプログラムを実行すると、decide_direction()内のｘとｙが意図したとおりに変化しません。教えてもらえませんか？
このコードを実行したときに出力されるｙの値を2, 4, 6, 8 ... としたいのですが、2, 2, 2, 2, ... と値が変わらないです。

#include <iostream>
#include <ctime>
/*
 *棒倒し法
 */

class maze {
  int wall_info[77][77] = {{0}};
  int x, y;
public:
  maze();
  void make_wall();
  int decide_direction();
  int wall_check();
  void show();
};

maze::maze()
{
  //make wall
  for (y = 0; y <= 76; y++) {
    for (x = 0; x <= 76; x++) {
      if (y == 0 || x == 0 || y == 76 || x == 76) {
        wall_info[y][x] = 1;
      }if (y % 2 == 0 && x % 2 == 0) {
        wall_info[y][x] = 1;
      }
    }
  }
  y = 2;
  x = 2;
}

void maze::make_wall()
{
  while (y < 76) {
    while (x < 76) {
      if (decide_direction() == 4) {
        wall_info[y - 1][x] = 1;
      }
      else if (decide_direction() == 3) {
        wall_info[y][x - 1] = 1;
      }
      else if (decide_direction() == 2) {
        wall_info[y + 1][x] = 1;
      }
      else {
        wall_info[y][x + 1] = 1;
      }
      x += 2;
    }
    y += 2;
    // std::cout << y << std::endl;
  }
}

int maze::decide_direction()
{
  int direction = 0;
  int check;

  std::cout << y << std::endl;

  if (y <=  2) {
    check = wall_check();
    if (wall_check() == -1) {
      direction = rand() % 3 + 1;
    }
    else {
      direction = rand() % 4 + 1;
    }
  }
  else {
    if (wall_check() == -1) {
      direction = rand() % 2 + 1;
    }
    else {
      direction = rand() % 3 + 1;
    }
  }

  return direction;
}

int maze::wall_check()
{
  if (wall_info[y][x - 1] == 1) {
    return -1;
  }
  else {
    return 0;
  }
}

void maze::show()
{
  for (int i = 0; i <= 76; i++) {
    for (int j = 0; j <= 76; j++) {
      if (wall_info[i][j] == 1) {
        std::cout << "■ ";
      }
      else {
        std::cout << "□ ";
      }
    }
    std::cout << std::endl;
  }
}

int main()
{
  srand((unsigned)time(NULL));
  maze maze;

  maze.make_wall();
  maze.show();

  return 0;
}