Inconsistent Media Playback (sometimes works, sometimes doesn't)

I’m using the Media Framwork in 4.20 to do video playback on an object through a texture. I’m pretty sure things are set-up properly (and following the documentation).

However - I get inconsistent playback results. Sometimes it works, sometimes it doesn’t. I can’t see any reason why playback won’t work each time, but not sure if there are troubleshooting for these types of issues.