As a copywriter (a person who writes headlines and stuff, not a person who applies for copyright protection), I find it works best when I have good leadership. Words have to be so specific to be meaningful and interesting (since words are testable; design…not as much). This means that when there’s not a good brief and a single goal for the piece, I have to play it safe with the words, which means it’s less engaging/penetrating copy. In other words, it starts to sound like an advertisement (which is almost never what you want).
So if you’re working with a copywriter who’s done good work before they showed up at your door, consider the fact that maybe you’re not providing specific enough guidance about what you want him or her to say for you.
One way to get more control over this process is to write a creative brief for your creatives. It tells them some very important information about the job, but it’s still pretty simple. If you want a copy of mine (complete with notes on how to use it), respond, and then give me a way to send it to you (for free, of course).