Why is the thumbnail size not updating?

Question

WordPress

Hello.

Created a new thumbnail size: Display in the template I realized that after creating a new thumbnail size, the pictures are not updated. I decided to use the Regenerate Thumbnails plugin . But the picture was not updated, as it was in 3 sizes, and remained. Maybe the fact is that the initial image size is smaller than the set one?
add_image_size( 'post', 540, 250, true );

<?php the_post_thumbnail('post') ?>