It has happened too often that some of my subfolders get out of sync. They just aren't replicated identically on all devices. I will have folders created but nothing inside of them on some devices, while there are hundreds of files on the original device where the files were created.
I don't know why this happens and it shouldn't happen. However once the problem is discovered, it should be easily to remedy. Add a Force Rescan or Validate or Resync or some type of catch-all error-checking button which will validate if a sync is proper and complete on all devices and immediately addresses the problem if they are not.
It could be done for an entire shared folder -- but also just to have the option for a subfolder would be ideal since presumably it would be faster.
I have been asking for this for over two years. It seems like an obvious inclusion and it is certainly necessary based on how many times my devices have failed to have fully proper synced subfolders.