csrf protection : error

[eluser]krishnakumar2k[/eluser]
Am using ci 2 .I have enabled csrf protection in config file.

Code:

$config['csrf_protection'] = TRUE;
$config['csrf_token_name'] = 'sitename';
$config['csrf_cookie_name'] = 'sitename';
$config['csrf_expire'] = 7200;

Am using codeigniter forms only.Some times i used to get the error "you are not authorized to perform this action" wile submitting forms or login.If i refresh and try angin then every thing works fine.Why is this happening ?.