files wouldn't play on internal media player, but would play through Jellyfin app

I have two files of Shark Night 3D, One MKV and One ISO. The internal player just kept saying unable to play video last night and I knew both were good. I opened up the Jellyfin app which plays through the internal player and the video played fine with flawless 3D. I've had this happen with a number of files now that it says can't play video. This is the first time I tried to play with Jellyfin instead, I just assumed the files were bad before.