Unable to ssh to machine. Outbound ssh works. sshd is running on port 22

Unable to ssh to machine. Outbound ssh works. sshd is running on port 22

I have an Ubuntu 12.10 machine on my LAN to which all ssh connections time out. Let's call it 'F' (for fail!)
Two other machines 'A' and 'B' on the same LAN can connect to each other. I can also ssh from F to A. So I don't believe there is a network config issue. The problem seems to be confined to machine F.
Following various other forums answers, I've verified and tried the following.
Verify sshd is running and listening on port 22:
F$ ps -A | grep sshd
853 ?        00:00:00 sshd

F$ sudo ss -lnp | grep sshd
LISTEN     0      128                      :::22                      :::*      users:(("sshd",4244,4))
LISTEN     0      128                       *:22                       *:*      users:(("sshd",4244,3))

F$ sudo lsof -i | grep ssh
sshd      4244    root    3u  IPv4  53321      0t0  TCP *:ssh (LISTEN)
sshd      4244    root    4u  IPv6  53323      0t0  TCP *:ssh (LISTEN)
ssh       5244 michael    3u  IPv4  82208      0t0  TCP localhost:40209->cubebot:ssh (ESTABLISHED)
sshd      5245    root    3u  IPv4  83301      0t0  TCP cubebot:ssh->localhost:40209 (ESTABLISHED)
sshd      5362 michael    3u  IPv4  83301      0t0  TCP cubebot:ssh->localhost:40209 (ESTABLISHED)

F$ netstat -nat | grep 22
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN    
tcp        0      0 192.168.1.28:41782      74.125.225.209:443      ESTABLISHED
tcp        0      0 192.168.1.28:47576      74.125.142.125:5222     ESTABLISHED
tcp        0      0 192.168.1.28:54925      91.189.89.122:443       ESTABLISHED
tcp        0      0 192.168.1.28:54762      74.125.225.167:443      ESTABLISHED
tcp        0      0 192.168.1.28:48473      74.125.225.181:443      ESTABLISHED
tcp        0      0 192.168.1.28:36177      74.125.225.198:443      ESTABLISHED
tcp6       0      0 :::22