The onboard credit has not been showing up in the account profile or payment history section for about a month now. I think it is more then a simple glitch, so definitely call Holland America; don't wait for them to fix it.
If it is still available, the chocolate factory tour in Barbados was a fun tour that I booked online before we sailed and then took a taxi to Mount Gay Rum in 2016. Made our own island tour for the day!