Help! Having some problems with the Plex app on my TV!

For some reason playing high quality MKV content doesn't seem to work at all. Other files work without a problem. It's as if it doesn't have enough bandwidth to stream the files which is nonsense as the android TV Video app seems to stream them fine.

Reasons for using Plex android TV app over others:

-Better movie browsing as I get the whole alphabet that I can use to browse quickly instead of making my thumb hurt swiping down on the silly remote

-Much easier to fast forward and rewind when needed again without making my thumb hurt lol

-Better quality than Kodi and no bad judder or slowdowns unlike Kodi

-Proper meta data with trailers

-Ability to resume which doesn't always work well on Kodi/Video app

My setup is a Synology media server running Plex and then connected to the Sony TV via 1200mbps homeplugs for a solid connection.

Any advice much appreciated.