Persistence is not happening

We have 60+ node pool configured for sticky session but requestors are moving between nodes. Here is persistence cookie config

ltm persistence cookie pr_persist_cookie_120_timeout {
    always-send enabled
    app-service none
    defaults-from cookie
    expiration 0
    timeout 180
}

We have a I-rule to handle logout scenario.

tm rule irule_prpc_logout {
    when HTTP_REQUEST {
         if {[HTTP::uri] contains "EndSession"}{
             set logout 1
         } else { set logout 0
        }
    }
    when HTTP_RESPONSE {
      if { $logout == 1 }{
           HTTP::cookie expires "BIGipServerprpc_pooll" 1 absolute
         }
    }
}

Any help really appreciated.