A too late reply for the OP, but wanted to say that I had a similar situation where my credit had not posted prior to sailing. I was able to go to Guest Services while on board and they gave me a customer service email address that was specific for guests of that ship. I had a screenshot of my proof of ownership and sent that in the middle of my cruise and received the credit in about an hour.
So just coming to say that it’s not too late even if you are on your trip!