apache 2.2 - WordPress can't install plugin even file permission is correct

I have apache2 server, of course apache2 is running on www-data account.

All my WordPress files is own by root:webmaster, and have the g+w permission.

Three of the accounts in webmaster group are www-data, sftp_www, and root itself.

The permission thing seems to be really, really good. Here are the copied text from the terminal, the same one with the screenshot above.

root@srakrn:/var/www/html/blog/wp-content# ls -l
total 20
-rw-rw-r-- 1 root     webmaster   28 Jan  8  2012 index.php
drwxrwsr-x 4 root     webmaster 4096 Jun  5 06:38 plugins
drwxrwsr-x 5 root     webmaster 4096 May  6 18:33 themes
drwxrwsr-x 2 root     webmaster 4096 Jun  5 06:38 upgrade
drwxrwsr-x 3 www-data webmaster 4096 Jun  5 08:55 uploads
root@srakrn:/var/www/html/blog/wp-content# groups www-data
www-data : www-data webmaster

This is what WordPress has asked: the FTP password. Usually WordPress won't ask for FTP password if the directory is writable by WordPress.

So, even the plugin folder is writable by the account www-data, why can't it install plugin files?