add_image_size not working

I am trying to resize an image using either the following code:

add_image_size('poster-full', 450, 9999, false);
add_image_size('poster-full', 450);

I need it to be resized widthwise, and height should be proportional.
When, I go and Run the Regenerate Thumbnails Plugin, i see that the image is resized correctly. For both the above cases.

But, when I do
the_post_thumbnail('poster-full'), The Orginal Image shows up, and not the resized one. I manually searched the directory. The resized Image exists.

Solutions Collecting From Web of "add_image_size not working"

For this to work, you must have added the thumbnail support. You can use the following code in functions.php to add the support

add_theme_support( ‘post-thumbnails’ );
add_image_size('poster-full', 450, 9999, false);

and to display the image:

if ( has_post_thumbnail() ) {
 echo "No thumbnail";