It might not be loading your thumbnail.
Also, make sure you put the video in the right field.
Alternative is always to just put it in a kind 1 or 22 and then tag it.
Discussion
I was having trouble getting bunker to work to log in yesterday. Will try again today.
Okay, so when I 'Upload File', it fails. Not sure what I am doing wrong there. But I uploaded directly to nostr build then linked 'Media URL', and it worked perfect 🎉
A bit confusing that Add Image also accepts Video, then conveniently auto-changes Note Kind for you. At first, I was changing Note Kind before adding until I realized it was unnecessary. Might be nice to 'Add Media File' and list all formats for Image and Video?
Yeah, that's not very intuitive.