PHP $_SESSION variable not showing value

I have written a text in the page title with the name of the logged in user. It worked for a while, but when I went to do something else, it just stopped. It 's like $_SESSION['ime'] holds a null value; I'm sure I'm not hitting anything related to this.

Here is some code: my main php file:

session_start();

// initializing variables
$username = "";
$email    = "";
$errors = array(); 

// connect to the database
$db = mysqli_connect('localhost', 'root', '', 'audiprojekat');

// REGISTER USER
if (isset($_POST['reg_user'])) {
  $vrsta = "KO";
  $ime = mysqli_real_escape_string($db, $_POST['ime']);
  $prezime = mysqli_real_escape_string($db, $_POST['prezime']);
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $email = mysqli_real_escape_string($db, $_POST['email']);
  $password = mysqli_real_escape_string($db, $_POST['password']);



  // first check the database to make sure 
  // a user does not already exist with the same username and/or email
  $user_check_query = "SELECT * FROM registrovani WHERE username='$username' OR email='$email' LIMIT 1";
  $result = mysqli_query($db, $user_check_query);
  $user = mysqli_fetch_assoc($result);

  if ($user) { // if user exists
    if ($user['username'] === $username) {
      array_push($errors, "Username already exists");
    }

    if ($user['email'] === $email) {
      array_push($errors, "email already exists");
    }
  }

  // Finally, register user if there are no errors in the form
  if (count($errors) == 0) {
    $password = md5($password);//encrypt the password before saving in the database

    $query = "INSERT INTO registrovani (ime, prezime, username, email, password, vrsta) 
              VALUES('$ime','$prezime','$username', '$email', '$password', '$vrsta')";
    mysqli_query($db, $query);
    $_SESSION['username'] = $username;
    $_SESSION['ime'] = $ime;
    $_SESSION['success'] = "You are now logged in";
    header('location: user.php');
  }
}

if (isset($_POST['login_user'])) {
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $password = mysqli_real_escape_string($db, $_POST['password']);

  if (empty($username)) {
    array_push($errors, "Username is required");
  }
  if (empty($password)) {
    array_push($errors, "Password is required");
  }

  if (count($errors) == 0) {
    $password = md5($password);
    $query = "SELECT * FROM registrovani WHERE username='$username' AND password='$password'";
    $results = mysqli_query($db, $query);
    if (mysqli_num_rows($results) == 1) {
      $_SESSION['username'] = $username;
      $_SESSION['ime'] = $ime;
      $_SESSION['email'] = $email;
      $_SESSION['success'] = "You are now logged in";
      header('location: user.php');
    }else {
        array_push($errors, "Wrong username/password combination");
    }
  }
}

?>

Here is the html page:

<?php include('server.php'); ?>
<p id="user_cont" style="color: white;  margin-left: 80%; margin-top: 0; margin-bottom: 0; width: 20%; background-color: black; padding: .5%; font-family: 'Rajdhani', sans-serif; font-size: 140%; border-left: thick red solid;">
                <?php
                    if(isset($_SESSION['username'])){
                        echo $_SESSION['ime'];
                        echo " <a href='logout.php' style='color: red; text-decoration: none;'>Odjavite se</a>";
                    }
                    else echo " <a href='login.php' style='color: white; text-decoration: none;'>Prijavite se</a>";

                ?>

            </p>

$_SESSION['username'] and $_SESSION['password'] are ok, but $_SESSION['ime'], $_SESSION['prezime'], $_SESSION['email'] are empty.

I checked the mySQL database and everything is written correctly.