I am a TA and if I see a rate went down on a sailing that a client is on I do notify them and see what I can do with the cruise line. Also if a client calls and says the rates went down I do see what I can do for them. But you have realize, as rude as this may sound, a travel agent cannot stay on top of every client who booked a cruise and see if the rate went down each day. Unfortunately the cruise line does not notify us (of course not because they want to make money) If I did that I would never leave the office! So as Geri617 was saying it is wise to check yourself and notify your agent.
If the rate went down after final payment was placed and Carnival cannot refund some back to you then you really should get a sipboard credit or an upgrade especially if the price drop was significant