Run a tcpdump on the server vlan, filtering for the self-IP address of the active LTM and one of the servers to which the monitor is applied.
You should be able to see the request & response on the wire. If you don't see both, check L2/L3 path (arp table, routing). If you see both, you might be able to see the problem (server error, redirect, HTTP version issue...)
If that doesn't help you resolve it, post back with your monitor config (b monitor monitorname list) and tcpdump of one request / response & maybe we can help further.
/d