Just because a problem may have been ongoing for several weeks or even months, unless one knows the extent of the problem I wouldn't say the cruise line should have fixed it by now. When dealing with marine propulsion systems you don't find warehouses with spare diesel engines, screws (propellers), and propeller shafts sitting on the shelves. Many things have to be manufactured per order, because they are custom fitted to a specific ship or have to be redesigned for replacement because of structural variances a hull may have experienced over time.
Then, as previously mentioned, you have to schedule shipyard time to effect the repairs. It isn't like the tour bus broke down and they pulled into Pep Boys to get a new fan belt. Some problems develop over time, despite proper maintenance. It could be a manufacturing defect that was fine for years and then suddenly manifested itself. Shafts are rarely "true" and have bowing which is compensated for by bearings. Bowing can worsen over time, to the point that RPMs have to be reduced or it could damage the reduction gears and engines, or wipe the bearings. There are too many variables and scenarios involved.