I used Amethyst to post this. Maybe it's your connection or the length of the video. But let's wait for other people to give you an answer.
Discussion
it's 4.30 minutes long, although I've seen nostr:npub1qny3tkh0acurzla8x3zy4nhrjz5zd8l9sy9jys09umwng00manysew95gx sharing long format vids. I've just downloaded Olas to see if that was an option but the UI is glitchy/buggy.
What version of amethyst are you using? And what file host when uploading the videos?
Nostr.build has a 50mb limit.
Try using nostr download one - it worked for my huge Pokémon match video.