"post_thumbnail" Function not working, images sizes aren't changing.

Hi guys,
My problem is that I’m trying to use the_post_thumbnail() function to display smaller images of the featured image on my index page to improve page load times. I’ve used this function successfully on previous websites but for this one it doesn’t want to work. The demo is at https://entecho.robmckaydesign.com . I’ve got the function file set up and I’ve taken it back to the basic set-up to try and find what is wrong. So in my functions page I have

if (function_exists('add_theme_support')) {
    add_theme_support('post-thumbnails');
}
set_post_thumbnail_size( 50, 50 );

just as an attempt to see if anything works. Yet all the times I use
<?php the_post_thumbnail(); ?>
I just shows the full size image but shrunk down to size due to my css.
I’ve tried using the regenerate thumbnails plugin every time i made a change but I still cant work it out.

Thanks for any help and I can provide any additional information you may need.