Revise the IP and port and syslog protocol configuration on both the client and server side. When the IP is missing from the source definition, then syslog-ng will listen on all interfaces. When there is any IP address, then the source will use that IP address found on the server.
In case of yes, configure SELinux/Firewall to not block syslog-ng operations. SELinux: Using syslog-ng with SELinux in enforcing mode Firewall: Most probably the source port must be enabled in the firewall. For solution check the documentation of the firewall application.
Check the connection from the client to the server with ping and telnet.