I would start by capturing traffic on both SMC and SIEM server to see if SMC is sending any UDP 2055 traffic to SIEM IP and if SIEM is seeing that traffic come in. If you don't see any UDP 2055 traffic leaving SMC server, something on SMC side seems to be causing this. First thing I would check is that if there's FW software (e.g. iptables on Linux server) running that it's allowing the traffic, and that SMC server has valid route to reach the SIEM server. Log server traces files in <smc_home>/tmp folder should also tell more but if those needs to be analyzed, it would be best to open Service Request to NGFW support team.
Similarly if traffic leaves SMC and reaches SIEM server, but you don't see the logs in SIEM, check that SIEM server don't have local FW software dropping the packets. If that's not the case, you might want to open SR for SIEM support team.
And for your question about additional receiver, I doubt that this is needed as long as there's UDP 2055 connectivity from SMC to SIEM. Though I'm not SIEM expert so I can't say for sure.