How to enable validation for form created in form builder (Symfony2)?

I create and render my form using form builder but it seems that validation is disabled or work not properly. I keep my validation rules in validation.yml file. Method $form->isValid() always return true, and $form->getErrorsAsString() doesn't show any errors (only [FieldName1]: No errors, [FieldName2]: No errors ... etc).

I create form in this way:

$form = $this->createForm( new CategoryType(), new Category() );

Then I send it to a twig.

What could be the reason? How do I enable validation?

----- UPDATE -------------------------

I create a form in this way:

public function indexAction()
{
    return $this->render('MyBundle:MyView:index.html.twig', array(
        "form" => $this->createForm( new CategoryType(), new Category() )->createView()
    ));
}

CategoryType is very simple:

class CategoryType extends AbstractType {
    public function buildForm(FormBuilderInterface $builder, array $options) {

       $builder ->add('name', 'text')
                ->add('categoryId', 'entity', array(
                    'class' => 'MyBundle:Category',
                    'property' => 'name',
                ));
    }

    public function getName() {
        return 'my_form_name';
    }
}

And of course validation.yml:

MyBundle/Form/Type/CategoryType:
    properties:
    - name:
        - NotBlank: ~
        - Length:
            min: 3
            max: 30

MyBundle/Entity/Category:
    properties:
    - name:
        - NotBlank: ~
        - Length:
            min: 3
            max: 30

I do not know exactly which version should I use, but both of them doesn't work.