目前我有两个域名，都解析到同一个 IP，并且这个两个域名都是可以 ping 通的，两个域名假设是：a.com, b.com，下面是我两个站点的配置：

####################### a.com这个站点  #############################
upstream app1 {
  server unix:/tmp/unicorn.app1.sock fail_timeout=0;
}

server {
  server_name a.com;
  listen 80;
  root /var/www/apps/app1/current/public;

  location ^~ /assets/ {
    gzip_static on;
    expires max;
    add_header Cache-Control public;
  }

  try_files $uri/index.html $uri @user1;
  location @user2 {
    proxy_redirect     off;
    proxy_set_header   Host $host;
    proxy_set_header   X-Forwarded-Host $host;
    proxy_set_header   X-Forwarded-Server $host;
    proxy_set_header   X-Real-IP        $remote_addr;
    proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
    proxy_buffering    on;
    proxy_pass http://app1;
  }

  error_page 500 502 503 504 /500.html;
  client_max_body_size 4G;
  keepalive_timeout 10;
}

####################### b.com这个站点  #############################
upstream app2 {
  server unix:/tmp/unicorn.app2.sock fail_timeout=0;
}

server {
  server_name b.com;
  listen 80;
  root /var/www/apps/app2/current/public;

  location ^~ /assets/ {
    gzip_static on;
    expires max;
    add_header Cache-Control public;
  }

  try_files $uri/index.html $uri @user3;
  location @user4 {
    proxy_redirect     off;
    proxy_set_header   Host $host;
    proxy_set_header   X-Forwarded-Host $host;
    proxy_set_header   X-Forwarded-Server $host;
    proxy_set_header   X-Real-IP        $remote_addr;
    proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
    proxy_buffering    on;
    proxy_pass http://app2;
  }

  error_page 500 502 503 504 /500.html;
  client_max_body_size 4G;
  keepalive_timeout 10;
}

现在的问题是当我访问其中一个域名是，站点是能正常访问的，但是另一个访问不到，用 chrome 查看发现请求 assets 的时候处于 pending 状态， 然后过一会请求就失败了，请教一下，这种是什么问题呢？