PreUpdate not triggered

I have this method in my entity:

/**
     * @ORM\PreUpdate()
     * @ORM\PrePersist()
     */
    public function preStore() {
        if ($this->getPictureFile()) {
            $newFilename = sha1(mt_rand());
            $newFilename = $newFilename . '.' . ($this->getPictureFile()->guessExtension());
            $this->setPictureFilename($newFilename);
        }
    }

And when persisting objects everything works just perfectly but on update the method is not triggered at all, i tested it this way:

/**
     * @ORM\PreUpdate()
     * @ORM\PrePersist()
     */
    public function preStore() { var_dump('asdasdasdadsdasdas');
        if ($this->getPictureFile()) {
            $newFilename = sha1(mt_rand());
            $newFilename = $newFilename . '.' . ($this->getPictureFile()->guessExtension());
            $this->setPictureFilename($newFilename);
        }
    }

And in persisting var_dump works, but when I update the object - it doesn't. Why?