Adding "-vvv" to the ssh command line makes it spit out lots of useful debugging information.
If you see something like the following in the output:
debug1: Unspecified GSS failure. Minor code may provide more information Server not found in Kerberos database
check to see if you have an /etc/hosts file with the host that you're trying to reach in it. If there is an entry for this host, make sure that the fully-qualified domain name is listed first, before any aliases that you may be using.
Session freezes after ~5 min of inactivity
The likely cause is that you are trying to exchange Kerberos tickets and have none.
If it still doesn't work
See the Troubleshooting Kerberos page for more diagnostics. You may also send the output of your ssh command with the "'-vvv'" to hcoop-help and we'll try to figure things out from there.