i have just moved my ripped video library to a NAS so I can access it via smart tv's or Kodi

When viewing the files via Tv I can see the file folder structure and initially file icons which i presume to be a default icon provided by the tv (a grey icon of a frame of film). After a minute or two these disappear to leave a very faint dark box. Only one title actually displays an embedded icon of the film cover art.

I have tried searching for where artwork is stored in the film file (now mp4) but have no success. I am using easyTag on linux to do this. I have also tried embedding new artwork but get no success in getting it to display on screen (although it is saved in the file itself).

If i view the video files, in icon view, on a computer i can see the embedded artwork as icons.

What am i doing wrong and how can I get to see the graphic icons on the tv?

Incidentally if step through the virtual invisible tv icons I can see the file name ok.