NGINX: Unable to receive response from socket server

I use nginx-1.9.1 as a server to relay to socket servers. The socket server can receive the socket request and do sth., but the client is not able to receive response from the server. The configure of the nginx is:

stream {
upstream xxx {
    server ipxx:8800;
}
server {
    listen 188xx;
    proxy_pass xxx;
}
}

And the log shows:

client disconnected, bytes from/to client:2/0, bytes from/to upstream:0/2

I don't know why.