OpManager V12 has 2 different Syslog servers , one for capturing specific syslog messages based on the syslog rule configured for generating alarms in OpManager and another one for Firewall module for monitoring complete syslog traffic. Only one can be used at a time.
OpManager will listen to Syslog port 514 by default , if the Firewall device is sending syslogs to the server with port 514 , it will be occupied by OpManager, hence we can not see anything the Firewall module , you can change the port meant for OpManager from 514 to any dummy port so that 514 will be used for Firewall module.
To change the syslog port in OpManager , go to Settings > Monitoring > Monitors > More > Syslogs >Syslog port > change the port> Save.
To configure Syslog port for Firewall Analyzer inside OpManager , go to Settings > Firewall > Syslog server > Add >Profile name > configure port 514 > Save.
Restart the Opmanager service and verify if you are able to get syslog information .