cakephp session lost and sometimes refresh webpage it can be get

like login I write
      $this->Session->write('custpyzm',$verifyCode);
 and I submit then in users_controller I write
      $this->Session->read('custpyzm');
but sometimes it can't be getted; and if I refresh webpage sometime it can be getted sometimes can't ?
what's on earth ?help someone can tell me ,thinks very much

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