When I click on the thumbs up or thumbs down symbol nothing happens. How can I vote for a bad answer on here?

There's this one person that I've noticed always answers questions poorlyand not just poorly, rudely. It's obvious this person is only answering to aculate points and not to help people at all. I want to vote on that person's answers as a poor or bad answer but like I mentioned above, when I click onthe thumbs down or thumbs up symbol, nothing happens. Why is this?