You have gotten great information on this subject.
Kuki stated it very well as to how the process works.
And iuki added some wonderful additional information.
The only thing I can add is to say that most reputable agents will monitor your rates for any specials they can get for you. But even if they do, it's always a very good idea to monitor your own rates. As was stated, an agent simply cannot monitor every rate, every day, for every client - just physically impossible. So I suggest just looking at the cruise line's website once a week, especially on Tuesdays, to see what's going on.
But once you've found a good agent that you trust, stay with them as they will look after you and help build a good working relationship.