Server not seeing proxy - known good proxy - troubleshooting tips?

[mmacpuguy]

zabbix-proxy was able to be seen by another zabbix server.

Built a new zabbix appliance and pointed the zabbix proxy to it. Does not see the proxy. Double checked the ip & hostname in the zabbix_proxy_conf on the proxy. All is good.

Built another appliance-proxy, the server won't see it either.

Seems to be the server. Server is monitoring hosts OK.

Suggestions on how to troubleshoot this?

Thanks