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"