Currently i have these lines in functions.php and my home page respectively:

    add_theme_support( 'post-thumbnails' );
    add_image_size( 'square', 252, 252, true );
//

    if ( has_post_thumbnail()):
          the_post_thumbnail( 'square' );
        endif; 

Then regenerate thumbnails plugin resizes images to 252 x 252. 

How do I display cropped thumbnail images and not featured?

Just to add , im using ACF custom fields plugin and part of that tinkers with the featured iamges, so it maybe possibly re-writing native  logic in how to handle images. If anyone has been in a similar situation, please let me know if ou found a work around. Thanks!