When I had a somewhat related situation where I spotted a price drop, I contacted the agency where I'd bought the cruise (VacationsToGo). In my case it was a lowly inside cabin. They wouldn't give me any money back, but instead upgraded me to a window cabin and gave me some on-board-credit. I was happy with the deal as the original price I'd paid was already excellent. I know the agent went back to the cruise line to sort things out, but I didn't deal with them. Another later cruise I spotted a price drop and thought I'd go for something similar. No luck this time, the agent explained to me, as I'd passed the full-payment date.

I have to assume the results are pretty arbitrary and depend on all sorts of things. Some of which we will never know.

Hope someone has experience more specifically relate to your situation. But you might check the full-payment date and be sure to act before that.