It will help if you can specify the Hibernate version that you are using and also describe your use case in a bit more details. :-)
It is Hibernate 2.1.6 and we have 2 servers each running a web application (using Sticky sessions on a load balancer to handle session issues) . Both are running treecache.
When starting they find each other (according to logs) and various tests do imply they are clustering fine.
Then as a test if I stop one of them the other starts complaining about not being able to get a lock. This keeps occuring until the other server is restarted and then it all works.
I used a treecache.xml that came with Hibernate, and compared to the one in hibernate in action and the JBoss clustering docs.