Some people, including myself, experience that ProFTP is stopped regularly. It seems that this happens when log rotation is active.

In the system log you'll see errors like:

... ProFTPD killed (signal 15)
... ProFTPD 1.3.5rc3 standalone mode SHUTDOWN

The problem seems to be that ProFTPD doesn't stop in time to be restarted. It's simple to fix this error in "/etc/init.d/proftpd".
Search for start-stop-daemon --stop --signal $SIGNAL --quiet --pidfile "$PIDFILE"

And replace it with start-stop-daemon --stop --signal $SIGNAL --retry 1 --quiet --pidfile "$PIDFILE"