HTTPS monitors not getting the members up

You can use the curl command from a bash prompt to simulate the monitor and see the response that the server is providing. This is sometimes faster than enabling monitor logging or using a packet capture to diagnose monitor issues.

For example, to send your current Host A monitor run the following from a bash prompt:

curl -kvv -H "Connection: Close" -H "Host: a.xyz.com.au" https://[pool member ip]/health/prospect

You can then modify the command to simulate changes to your monitor config and view the response from the server.

You noted that the monitors are identical other than the send string, but can you confirm the configuration of each monitor's recv string? If the pools are truly identical (same members across both pools), then I suspect you're getting at least some response to both monitors given success on pool B. An https monitor with no recv string would mark the pool up with any response from the member. Confirm you have an appropriate recv string for the failing monitor, and use the curl command above to review the server's actual response. This should provide some clues to narrow the issue.