**This issue has been resolved in ArcGIS 9.3.1 Service Pack 2.**
In ArcGIS Server, Failover and Round-robin are two methods for configuring multiple Server Object Manager (SOM) machines.
After applying Service Pack 1 to ArcGIS Server 9.3.1 for the Java Platform, unexpected behavior is experienced with Round-robin and/or Failover in the Server environments.
With Round-robin, the request is always sent to the secondary host.
With Failover, if a failure occurs with the primary host SOM machine, the initial failover from the primary to a secondary SOM host works as expected. However, if the primary host machine becomes available again, the requests continue to go to the secondary host. If the secondary host goes down, then the requests do not failover back to the primary host machine.
Service Pack 1 has introduced a bug into the Round-robin and Failover mechanism.
For reference the bug number and synopsis are NIM052853 - Round-robin or failover configuration do not work as expected.
If the version 9.3.1 Service Pack 1 has already been installed and this issue is occurring, please contact ESRI Technical Support.