Custom image sizes are not used

The function.php:

if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions   
}

if ( function_exists( 'add_image_size' ) ) { 
    add_image_size('homepage', 704, 396, true);
    add_image_size('articles', 1174, 660, true);
    add_image_size('in-post', 568, 320, true);
    add_image_size('featured-post', 1280, 683, true);
}

To make it appear in .php-file:

<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'in-post' ); } ?>

That would be my suggestion.