Database problems

Hi I have just set up my elgg site, however, when I display some contents, like the wire posts, it says

Elgg couldn't connect to the database using the given credentials.

I checked the settings.php in engine folder, I believe I have got everything:

// Database username
$CONFIG->dbuser = 'my_db_username';

// Database password
$CONFIG->dbpass = 'my_db_psw';

// Database name
$CONFIG->dbname = 'my_db_name';

// Database server
// (For most configurations, you can leave this as 'localhost')
$CONFIG->dbhost = 'dbhost';

// Database table prefix
// If you're sharing a database with other applications, you will want to use this
// to differentiate Elgg's tables.
$CONFIG->dbprefix = 'dbprefix_';

Please give me some advice.