wp_get_attachment_image_url is not returning the good image size. The function need the name of the image size or a valid image size (1200 x 9999 is not a valid size, just the the way to tell add_image_size cropping definition).
Then the scrapper will be able to scrappe your content. If think there is a minimum and maximum size, and your size seems to be a little bit heavy.
Hope it helps you.