AC3 5.1 working for some files, for others only Stereo

My setup: Plex App PS4 - HDMI - Philips HT Audio - HDMI TV
PS4 Sound settings: HDMI and PCM
PS4 Plex App: 3.13.9 WebMAF 01.55
Settings: AC3 enabled with Direct Stream and Direct Play under Video checked

Certain Movies with AC3 5.1 have 5.1 output, while others even though shown as Dolby 5.1 by plex app, gives only stereo output. please help!