

There’s probably a codec issue where Plex thinks something can direct play but it can’t. I’ve had similar issues a few times over the years especially with audio codecs like EAC3 for example. Easiest thing to do is to just find a different version of the file and try to avoid that codec for the time being.
You can force a transcode by dropping the quality of the video in the player settings. Perhaps something is misconfigured with HW transcoding and that’s the actual reason why your videos aren’t playing (and not the codec issue I mentioned in my other comment).