load balancing citrix servers

Hamish, Thanks for the clear explanation, but What I have a hard time figuring out is that in my production Citrix environment some apps come up no problem through the Web Interface. But in my test environment (routed to the same back end Citrix production farm), I'm using the F5 to proxy, only some apps come up. Others start to initiate to launch and then say "Unable to launch your application. Contact your helpdesk with the following information: Cannot connect to the Citrix XenApp server. Network issues are preventing your connection. Please try again. If the problem persists, please call your helpdesk".