Django signals doesn't work when creating model instance implicitly

Question

I can't find out why my signal does not work when the instance (of proxy model) is created implicitely by third app.

I use a third party app Django-Quiz in my project. There is a model called Sitting. Since I want to see the table in admin, I had to create a proxy model QuizSittingProxy, to be able to set __str__ and __unicode__ methods for Sitting.

Each time when User takes a quiz, the new Sitting object is created. Respectively QuizSittingProxy object is created too.

class QuizSittingProxy(Sitting):
    """
    Holds information about taken Language Tests (in admin)
    """
    class Meta:
        proxy = True

    def __str__(self):
        return '{}: {}, score: {}'.format(self.user.username, self.quiz.title, self.current_score)

I want to assign result of the quiz to current User so I've created a signal.

The problem is, that this signal works only if I create a QuizSittingProxy object manually in admin.

Worth to say that it is created everytime User takes a quiz.

@receiver(post_save,sender=QuizSittingProxy)
def assign_level(sender, instance, created, **kwargs):
    print 'QUIZSITTING POST SAVE'
    # user = instance.user
    # quiz = instance.quiz
    # score = len(instance.user_answers.keys()) / instance.current_score

Do you know where is the problem?