$form->isValid says it is invalid, but my form does not show any error message - Zend Framework 2

Im having a problem with the addAction in my CRUD application. On the controller the logic does not pass the $form->isValid() verification but the form does not show any error message.

I tried with this (Thanks Sam):

foreach($form->get('product')->getElements() as $el)
{
    echo $el->getName()." = ".$el->getValue()." > ".$el->getMessages()." <br/>";
}

That only show the name and value of the field, but not the error message.

I've tried letting the form totally blank and it fire "Value is required and can't be empty" error messages, but then, i fill each field one by one until i don't get more error messages but the form still invalid.

My form has a Product Fieldset as a base fieldset and a submit button. Inside Product Fieldset i have an ID field, a name field, a price field and a Brand Fieldset. Inside my Brand Fieldset i have a id field. Like this:

ProductForm:

class ProductForm extends Form
{

    public function init()
    {
        // we want to ignore the name passed
        parent::__construct('product');
        $this->setName('product');
        $this->setAttribute('method', 'post');

        $this->add(array(
               'name' => 'product',
                'type' => 'Administrador\Form\ProductFieldset',
                'options' => array(
                        'use_as_base_fieldset' => true
                ),
        ));
        $this->add(array(
                'name' => 'submit',
                'type' => 'Submit',
                'attributes' => array(
                        'value' => 'Add',
                        'id' => 'submitbutton',
                ),
        ));
    }
}

ProductFieldset:

class ProductFieldset extends Fieldset implements ServiceLocatorAwareInterface
{

    protected $serviceLocator;

    function __construct($name = null)
    {
        parent::