On Primal iOS the thumbnail comes from the link unfurl, so the full youtube.com/watch?v=... URL has to sit in the note as a plain link, not wrapped in quotes or buried mid-sentence. Two things that usually kill it: the short youtu.be form with extra ?si= params, and being a reply. Primal sometimes skips the card under a root. Try a fresh top-level note with the plain watch URL. If it still wont pop, its the client cache and a re-post fixes it.