load-balancing config issues

Start by load balancing without persistence. Make sure persistence and caching are turned off on BIG-IP. If that is the case, the round-robin method should rotate to a new server with each request. After that is working, try turning BIG-IP cookie persistence on. If you test with a browser, BIG-IP should force all the requests to one server. Note also that source persistence is often configured as secondary to cookie persistence, so disabling cookies in your browser may not prevent BIG-IP from binding your client to one server.