问题请教：nginx负载均衡，用posthttp循环提交随机出现未响应

问题请教：nginx负载均衡，4台机器，基于apache服务器，用posthttp向nginx循环发送请求随机出现未响应。
nginx访问日志access.log中未出现记录
求大神，何解？
[09/Apr/2021:10:38:54 +0800] "POST /****.php HTTP/1.1" 200 0 "-" 10.******:80
[09/Apr/2021:10:39:55 +0800] "POST /****.php HTTP/1.1" 200 0 "-" 11.******:80
[09/Apr/2021:10:40:54 +0800] "POST /****.php HTTP/1.1" 200 0 "-" 12.******:80
[09/Apr/2021:11:15:05 +0800] "POST /****.php HTTP/1.1" 200 0 "-" 13.******:80
丢失了一个！
nginx配置如下：
events {
worker_connections 1024;
}
http {
include mime.types;
default_type application/octet-stream;
log_format main '$remote_addr - [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" $upstream_addr';
access_log logs/access.main.log main;
sendfile on;
#tcp_nopush on;
#keepalive_timeout 0;
keepalive_timeout 65;
underscores_in_headers on;
gzip on;
upstream mySever{
server 10.*****:80;
server 11.*****:80
server 12.*****:80
server 13.*****:80
}
server {
listen 9999;
server_name mySever;
location / {
root "Z:/My/web";
index index.php;
}
location ~* \.php$ {
proxy_ignore_client_abort on;
proxy_connect_timeout 65s;
proxy_send_timeout 3600s;
proxy_read_timeout 3600s;
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
add_header Kss-Upstream $upstream_addr;
proxy_next_upstream error timeout http_500 non_idempotent ;
proxy_pass http://myserver;
}