You may have reached your quota of watches.
To find your current limit, type this in your terminal:
cat /proc/sys/fs/inotify/max_user_watches
Which is typically 8192 by default.
To increase your limit, type this:
sudo sysctl fs.inotify.max_user_watches=16384
Then restart django.
To permanently set this limit, type this:
echo 16384 | sudo tee -a /proc/sys/fs/inotify/max_user_watches