Are you able to post the config on the FTP VIP?
Do you have a TCP monitor that is enabled on the VIP and marking the both Pool members as UP?
How are the Pool members configured? Are they Round Robin or least conns, or do you have a ratio or priority set?
If you have access to the command line and you understood the vlan's then you could do a TCPDUMP on the ingress and egress vlans. Do you know how the traffic is flowing?
Are you source natting the traffic of passthough?
You can also do a generic TCPDUMP but you have to be careful, is the box in production?