A few things off the top of my head:
- Are there labor camps connected by road to the storage yard(s) with sandstone?
- Is there a ferry crossing involved with the stonemasons or labor camp? e.g. do the stonemasons or laborers have to use a ferry to get to the monument, or to the sandstone?
- Is there a predator on the map, such as crocodiles, hippos, etc.?
For the first one, verify that there is a road connection between the labor camp and the storage yard(s) with sandstone. There doesn't need to be a road to the monument site, but I'm fairly certain there needs to be a road between labor camps and the appropriate storage yards.
As for the ferry crossings and predators, if these are true, then it is possible that one of your stonemasons or laborers got stuck on the ferry, or killed by a predator. To check this, you can save your game, then delete and undo the delete of each labor camp in turn. It sounds like the stonemasons are fine, though, if you see them carving the monument. Do you actually see the stonemasons at the monument site? Or is it just the message that indicates they are carving it?
As a final resort of action, you can always save the game again and delete then undo the delete of your monument.