Look in your /etc/security/access.conf file. It may prohibit access for root on remote connections.
Look for the line:
%wheel ALL=(ALL) SETENV: ALL
It may have been reconfigured during a security fix.
Check if the above line looks like this instead:
%wheel LOCAL=(ALL) SETENV: ALL
Also, read the log in /var/log/messages and if you have it /var/log/secure. It may have recorded your su attempt when using ssh and indicate why su was rejected.