Issue with the TCP Load Balancing

Dear all,

I am the beginner with NGINX load balancing and i am facing with the problem.

In detail, I set the upstream servers as the Mosquitto service which is running on port 6677 . There are 2 mosquitto servers in the upstream server. One is the master, the other is backup.

After that,I run the command "kill -SIGSTOP PID" to hang the mosquitto service in the master server.
The issue is the Nginx load balancing does not switch the mosquitto connection to the backup server so that i can not get data as expected.

Please help for this case. I hope that if the service is hang in the master server, the connect will be switched to the backup server.

Thanks for your help and sorry about my poor english.

Here is my configuration:
upstream backend {
server 192.168.50.14:6677;
server 192.168.50.15:6677 backup;
}