Cannot get full thumbnail size using the_post_thumbnail

Try to add custom thumbnail size with unlimited(very big) hegiht and width in functions.php with following code:

add_theme_support( 'post-thumbnails' );
add_image_size( 'custom', 9999, 9999 );

It will not crop any images. So you can use it as the_post_thumbnail( 'custom' );

This method looks rough but you can use if other ways don’t work. And if you already have images, you can use regenerate thumbnails plugin.