DVR recordings in mkv format not playing on Android or Amazon fireTV

Hi. Having bought software to edit .ts files which now doesn’t work with the new .mkv files, I’ve just noticed that I also can’t play anything on either my Android phone or Amazon fireTV. Has anyone else had issues with the new format? Seems worse with HD 5.1 audio files.

First I haven’t had any issues. Also saying you can’t play does not make sense. Plex should transcode if needed. What is on the display when it fails. Is it trying to transcode?

Just a whirring ‘waiting’ symbol.