the_post_thumbnail('my_image_size') always outputs full image

Question

I'm developing a custom theme that uses several custom image sizes defined in my functions.php like this:

add_theme_support('post-thumbnails');
add_image_size('featured_thumb', 175, 120, true);

I then get posts from the database and output on a page like this:

$featured=get_posts();
if ($featured)
{
          global $post;
          foreach($featured as $post)
          {
              setup_postdata($post);
              the_post_thumbnail('featured_thumb');
          }
          wp_reset_postdata();
}

But this code always outputs a full sized image. I can't figure out why - everything else seems to work alright (the_title(), the_permalink() etc) and it does get the correct image but not in the size I requested.

Does anyone know under what circumstances the_post_thumbnail() will ignore my image size, or what I'm doing wrong?

Edit: using WP 3.4.1

Answer 1

Remember that custom image sizes are NOT applied retroactively to images already uploaded.

Use a plugin like Regenerate Thumbnails or similar to generate them.