nginx配置，多个子站均指向默认welcome to nginx页面

Question

nginx.conf

#user  nobody;
worker_processes  1;

#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;

pid        logs/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    #access_log  logs/access.log  main;


    sendfile        on;
    #tcp_nopush     on;

    #keepalive_timeout  0;
    keepalive_timeout  65;
    server_tokens off;
    gzip  on;

    server {
        listen       80;
        server_name  _;

        #charset koi8-r;
        server_name_in_redirect  off;
        access_log  /home/logs/www.abc.cn-access.log main;

        location / {
            root   /home/site/cn/abc/www;
            index  index.html index.htm;
        }

        location ~* .*\.(js|css)?$
        {
            expires 7d;
            access_log off;
        }

        location ~* .*\.(png|jpg|gif|jpeg|bmp|ico)?$
        {       
            expires 7d;
            access_log off;
        }

        location ~ /\.ht {
            deny  all;
        }
    }


    include /usr/local/nginx/vhost/*;

}

vhost下面有两个配置文件：
a.conf 和 b.conf
a.conf:

server {
        listen  80;
        server_name    a.abc.cn;
        access_log  /home/logs/a.abc.cn-access.log;

    location / {
        root   /home/site/cn/abc/a/public;
        index  index.php index.html index.htm;
    }

    location ~* .*\.(js|css)?$
    {
        expires 7d;
        access_log off;
    }

    location ~* .*\.(png|jpg|gif|jpeg|bmp|ico)?$
    {       
        expires 7d;
        access_log off;
    }

    location ~* .*\.(zip|rar|exe|msi|iso|gho|mp3|rmvb|mp4|wma|wmv|rm)?$
    {
        deny all;
    }
}

b.conf

server {
        listen  80;
        server_name    b.abc.cn;
        access_log  /home/logs/b.abc.cn-access.log;

    location / {
        root   /home/site/cn/abc/b/public;
        index  index.php index.html index.htm;
    }

    location ~* .*\.(js|css)?$
    {
        expires 7d;
        access_log off;
    }

    location ~* .*\.(png|jpg|gif|jpeg|bmp|ico)?$
    {       
        expires 7d;
        access_log off;
    }

    location ~* .*\.(zip|rar|exe|msi|iso|gho|mp3|rmvb|mp4|wma|wmv|rm)?$
    {
        deny all;
    } 

}

在/home/site/cn/abc/www，/home/site/cn/abc/a/public，/home/site/cn/abc/b/public下面均有index.html文件。。。但是访问这三个页面：www.abc.cn、a.abc.cn、b.abc.cn都是返回的nginx的默认页面welcome nginx。。。

请问我的配置文件哪里出了问题了么？

注：本来只有a.abc.cn和www.abc.cn的时候是可以正常访问的。但是加了一个b.abc.cn之后就出现了所有域名都指向默认页面的情况。
在增加b.abc.cn的时候，我只是在vhost中增加了一个b.conf的文件，其他均未改动。