[fb:request-form] invitation never arrive in wall friend

Question

Hi,

i try to create invite form in my fb app, using this code

<?php
// Prepare the invitation text that all invited users will receive.
$content = <<<FBML
<fb:name uid="$user_id" firstnameonly="true" shownetwork="false"/> wants to know what your Favorite Games are!
<fb:req-choice url="{$facebook->get_add_url()}" label="Add Favorite Games to your profile!"/>
FBML;
?>
<fb:request-form action="http://apps.facebook.com/green_test/" method="POST" invite="true" type="Favorite Games" content="<?php echo htmlentities($content);?>">
    <fb:multi-friend-selector max="20" actiontext="Here are your friends who haven't added Favorite Games to their profile. Invite them to share their Favorite Games today!" showborder="true" rows="5"></fb:request-form>

Invite form was rendered, and submit running well, but the invitation never arrive in my friend's wall. what should i do? is this normal? or maybe there's a delay?

thanks and regards :)