It's hard to tell from your description but given that your ports are 38080 and 38081 I'm guessing you are setting up your cluster on a single host.
If so then there is an easy way to see if the problem is your load balancer setup.
Simply run the app from a browser connecting directly to each instance
e.g.
http://<host>:38080/cluster.jsp
Do some requests that way - add attributes, etc.
Then do what we call "port-hopping on the same machine"
http://<host>:38081/cluster.jsp
If this works then high availability session persistence is working and you
can focus on your load balancer issues.
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