post_password_required() not working

Hi all,

I have added custom fields to the Media area of WP to add a password to an attachment (working), but when I enter the password WP is telling me that the the password is still required.

$wp_password_required = post_password_required($post->ID);
if($wp_password_required) :
	echo get_the_password_form();
else :
	get_template_part( 'loop', 'attachment' );
endif;

I’ve looked in the $_COOKIE variable, and the correct password is being assigned to the wp-postpass_#### key, but for some reason it does not like what is in there.

Am I doing something wrong?

Thanks.