On Linux, the Dropbox client or in general all applications are subject to a default file system limit regarding the number of directories and files an application can monitor for changes.
Dropbox sooner or later will notify you with the following warning:
“Unable to monitor the filesystem – Please run: echo 100000 | sudo tee /proc/sys/fs/inotify/max_user_watches and restart Dropbox to correct the problem.”
The Linux version of the Dropbox desktop application is limited from monitoring more than 10000 folders by default. Anything over that is not watched and, therefore, ignored when syncing. There’s an easy fix for this.
# increases the max-file-watch limit
echo fs.inotify.max_user_watches=100000 | sudo tee -a /etc/sysctl.conf
# to apply the changes without restarting
sudo sysctl -p
sudo dropbox stop
sudo dropbox start