why not work VideoPlayer in godot 3.5.1/linux/flathub
You probably need to provide more info, what's your code & scene tree like, what video container & formats you've tried...
Yeah, you'll have to post more information, namely about the codec used and the container. However, I've had some issues playing videos and was able to solve it by recompressing the video into MP4 or Theora using FFMPEG.