A lot sepends on where you purchased your cruise. Carnival, as well as other cruise lines, sell blocks of space to a travel agency. They then have to sell or lose. As they get desparate, they drop rates, to a level allowed by Carnival. If you purchased through Carnival, and they drop the rates, they will most times match it. If it drops elsewhere, don't expect them to match it. By checking the Carnival website on a daily basis, if you see a lower rate, call them.
Hope this helps.