Hi,
@sahdogra Perform the following:
1. Ensure HA state for the two boxes, is as expected, Active / Standby; if not, fix it.
2. Verify the state of all monitored interfaces; fix it, if not a expected.
3. From the Active device, ping the standby IPv4 address on each of the links configured with IPv4 addresses; whatever doesn't work, fix it (either no standby IPv4 is configured for that link, either there's a layer 2 issue and the two boxes don't see each other at layer 2 over that segment).
The above tests will validate the HA and control-plane readiness only, data plane can only be checked upon a failover.
Once all of the above tests have passed, it means everything should work upon a failover, so you can safely trigger a manual failover to test data plane as well (validate everything works as expected, afterwards failover back again).
You can make use of the following document as a guide:
https://www.cisco.com/c/en/us/support/docs/security/firepower-management-center/212699-configure-ftd-high-availability-on-firep.html
Thanks,
Cristian.