Ido,
You're certainly right about how the f5 will work, it will listen and then forward traffic to a pool member. First thing to check is that the monitors have marked the pool / node up. Any issues here and the traffic won't get forwarded, even if the origin server is up.
Do you see any connections hitting the VS? Check out the Statistics page in the GUI. If not then it's an issue from the firewall to the f5. If you do see connections then you need to check the route from the f5 to the origin server. Is the origin server IP on the same subset as a configured VLAN on the f5? If not it may need a static route setting up in my experience.
Tcpdump and / or wireshark should help you locate where the issue lies and f5 has the tracepath command as well.
Good luck...
N