FLAC file playback on Amazon Firestick PLEX app

I have all my music stored as FLAC (ripped using EAC) on my Media Server. PLEX app works brilliantly on our phones, laptops, tablets, my Yamaha receiver and the PLEX app on my Samsung TV. However, when trying to use PLEX app on my Firestick, all the album art and metadata show up absolutely fine but when I press play on a track I get nothing. The play button is illuminated orange but the track just doesn’t run. A few movies that I also have stored on the server do play back on the Firestick. Is this a file compatibility issue or am I missing something in the settings for either PLEX or the Firestick? Any help much appreciated.