It could be that your company may have a firewall that's blocking the website where the images are being pulled from (i.e. stock imagery). Please ask your IT to have this website whitelisted.
Or the image is now broken because it was pulled embedding a public URL link from a website. (This can be in result of a broken image, deleted image, changed URL link, etc.)
Tip: Download your photos directly and then re-upload them directly to your media library to avoid these situations.
If the images are showing up as blank but not broken in your media library, it just may be a white graphic (sitting on a white background) so it appears "hidden".