Does plex switch to an optimised version on it's own

Trying to play 720p mkv files on a Tivo Roamio and they completely refuse to play. I get the three dots and then the go back and try again. And they will not play but if I optimize the file in question and then try to play the original file again, it plays fine, so does plex use the optimized file on its own if the original will not play?