What format does it get converted into? Can you post a link that you got back (even in the comments)?
Each instance can configure pict-rs (the media subsystem, basically) a number of different ways or even disable videos entirely (or convert them to gifs, etc).
Regardless of format, Lemmy+Pict-rs isn’t really setup for videos longer than a few seconds (pict-rs has to transcode the upload, and there’s a hard timeout between Lemmy and pict-rs). Most people either upload to catbox and put the .mp4 link as the post URL or post a YT or Peer Tube link.