They are either still loading or the app could not find the video.
Discussion
I thought it might be relay load at peak times .. then I thought it might be something to do with the Amethyst picture NIP implementation .. then I thought it might be this 👇🏼
.. then I installed the latest version of Amethyst and everything seems to work fine 🤷🏼♂️
Hope this helps
🤙🏼