Images not showing in my feed

Hi,

For some reason, images are not shown in one of my rss feed.
This one is working : Actualité de l’éducation
This one is not working : Valises pédagogiques

Here is the list of actions I’ve already taken :
– desactivate the cache
– reset the permalinks
– reset the excerpt and the full length of articles
– desactivate most of my plugins
– added this code in my functions.php theme file :

// THIS INCLUDES THE THUMBNAIL IN OUR RSS FEED
function insertThumbnailRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) ){
$content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail' ) . '' . $content;
}
return $content;
}

add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');

And I still don’t find how the images don’t appear… Can some expert help me solving this ?
This problem is starting to get on my nerves ??

Thanks,
Benjamin