[CentOS] Odd issue with fail2ban

We're seeing on a few of our servers - and sometimes it's only
occasionally on some of those - where fail2ban's running happily, AFAIK,
but there's an attack (from China, Brazil, etc) on ssh, and they don't
seem to be banned; I see many, many sorries for wrong username or
password.

It *seems* to work again once restarted.

        mark