[wpv-post-featured-image size="custom-size"] not working?

Hi all,

Just wondering if I'm doing something wrong. I'm trying to use a custom image size in views which I've added to my themes functions.php file using
add_image_size( 'medium-cropped', 300, 225, true );

I'm inserting this in my view:
[wpv-post-featured-image size="medium-cropped"]

I seem to get an image tag with the custom-size class but loads the original (full size) image? Is there a way to make this work? Or will I need to modify one of the native sizes?

Thanks!

Carl