Post Thumbnails or Featured Images doesn't show on my main home page

Post Thumbnails or Featured Images doesn’t show on my main home page. But images appear on the other pages. Whenever I switch to another theme, especially “twentytwelve” theme, the featured image shows on home page.

I placed the code <?php the_post_thumbnail(); ?> in header.php. What will I do? I am new to WordPress. Thanks.