In CentOS 5.3 edit the file /etc/fail2ban/jail.conf for the [ssh-iptables] entry such that the line beginning with logpath... is altered to that shown in red below.
enabled = true
filter = sshd
action = iptables[name=SSH, port=ssh, protocol=tcp]
sendmail-whois[name=SSH, firstname.lastname@example.org, email@example.com]
logpath = /var/log/secure
maxretry = 4
You can view /var/log/secure before you make these changes to verify that SSH attempts are logged here.
After editing jail.conf don't forget to restart fail2ban with the command:
# service fail2ban restart
If you get an OK when fail2ban starts (i.e. the process has started), test it again and see if you're blocked after maxretry attempts.