each physical server has ten instance.
we have 3 physical server.
We solved the problem with multiple tests.
The problem was the session retention time.
The four test PCs consistently generated http requests with a fixed IP address.
However, with the session retention time of 300 seconds, the request was continuously transmitted only to the server that processed the initial request, and the load balancing could not be performed evenly.
With continuous continuous requests, the session retention time was constantly maintained at 300 seconds
This prevents load balancing to other servers until the server processing the request is stopped.
We solved the problem to equalize load balancing by releasing session time(setting 0).
thanks rob_carr