problem setting up a virtual server

As suggested by Neeeewbie,
from what you say, I'm guessing that you don't have F5 as your nodes' gateway.

So check the "Source address translation" setting:
If you set it to "Auto Map" F5 will present the packet with its own Floating IP to your nodes and they will know who to reply to.

If you want to use specific IPs for SNAT you have to create a SNAT pool.

If you leave the setting to "none" your nodes will see the packet coming from the "Client IP addr".
So if then node or gateway don't know the route you won't get any reply.

I hope I was clear and helpful.

Have a nice day.

Emil