Linux provides a fix number of filesystem watches. If you have some greedy daemon (like dropbox) running, chances are it uses all of them, and you cannot use tail -f any more.

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To increase the number of watches, put something like
fs.inotify.max_user_watches = 32768
into /etc/sysctl.conf. (default is 8192)

To update the setting right away, run
sudo sysctl -p /etc/sysctl.conf
afterwards.