wordpress thumbnail function not working

hi,
i was working with thumbnail image for my theme. i added the code

if ( function_exists( 'add_theme_support' ) ) {
  add_theme_support( 'post-thumbnails' );
}

in my function.php and then tried to call the function
<?php the_post_thumbnail(); ?>
in my theme’s loop but it returned error stating undefined function
I just tested the same post image in my post( i called the thumbnail size of the image) it showed the cropped image for 150 * 150, but if i change the media setting to change thumbnail image size it doesn’t change ( it still remain 150*150 ).

help me out guys.