Plex won’t play .mkv

I use plex and a plex media server on the same pc I watch the media on. I’ve never had this problem in the past but it will not play any .mkv files. I click play and it just goes to a black screen. They play fine in the vlc player just not in plex. It’s causing me to torrent and then convert everything to mp4. If anyone has any advice it would be much appreciated.