session failure

php version 7.1.4

phalcon version 3.1.2

controller file :

namespace Application\Controller;
use Phalcon\Mvc\Controller;

class TestController extends Controller{

    public function indexAction(){
        $this->session->set("user-name", "Michael");
        var_dump($this->session->get('user-name'));
    }

    public function aAction(){
        $a = $this->session->has("user-name")
        $b = $this->session->get("user-name");

        var_dump($a, $b);
    }
}

index.php(Entry file):

$di->set('session', function(){
    $session = new SessionFile();
    $session->start();
    return $session;
}, true);

access ‘/Test/index’:

string(7) "Michael"

access '/Test/a':

bool(false)
NULL

why?