Nginx Lets Encrypt Can't Redirect to www

My site works in both https access with www & non www. I've also changed the redirect from non www to www, and it works. All access is redirected to https://www.example.com

But, currently my website can't redirect to www even though with the same nginx default configure. Here's

server {
    add_header X-Frame-Options SAMEORIGIN;
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name example.com www.example.com;
    return 301 https://server_name$request_uri;}

server {
    listen 443 ssl default_server;
    listen [::]:443 ssl default_server;
    include snippets/ssl-example.com.conf;
    include snippets/ssl-params.conf;
    client_max_body_size 500M;

root /var/www/html/src/public;

    index index.php index.html index.htm index.nginx-debian.html;

    location / {
            try_files $uri $uri/ /index.php?$query_string;
    }

    location = /favicon.ico { log_not_found off; access_log off; }
    location = /robots.txt { log_not_found off; access_log off; allow all; }
    location ~* \.(css|gif|ico|jpeg|jpg|js|png)$ {
            expires max;
            log_not_found off;
    }

location ~ \.php$ {
            add_header X-Frame-Options SAMEORIGIN;
            try_files $uri =404;
            fastcgi_split_path_info ^(.+\.php)(/.+)$;
            fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
            fastcgi_index index.php;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_nam$
            include fastcgi_params;
    }

    location ~ /.well-known {
            allow all;
    }

    location ~ /\.ht {
            deny all;
        }

}

I'm using VPS debian 8.9 x64, nginx/1.6.2