If the snmp requests are reaching the FW and it's not responding, you should see syslogs.
There are reasons why it may not respond, mostly having to do with resource
utilization. Obviously, that should not normally be a problem on the standby FW.
If you don't already see these, you may have to enable standby logging.
If that still doesn't point you in the right direction, then you will need to collect a sniffer trace on the same subnet as the secondary FW along with accompanying syslogs from the same time period
