Confused with set_flashdata?

Question

CodeIgniter

Task: After successfully adding a comment, display the message with flash data
Adding comments via the Comment() method
In the Posts controller

Comment() {
<?php $this->session->set_flashdata('message', 'Комментарий успешно добавлен'); ?>
}

in view:

<?php echo $this->session->flashdata('message'); ?> 

  <form action="/posts/comment" method="post">
    
  </form>

why after sending a comment I have an empty, no message?