PLEXBMC with kodi won’t play the subtitle file next to the movie folder?

I installed plex server in my NAS (Synology) and using PLEXBMC with kodi in my HTPC.
Every think works perfect except the kodi won’t play the subtitle file next to the movie folder.
When I download the subtitle it read it but once you go out from the movie and play it again you can it see the subtitle only the embedded.
It’s like downloading the sub but not in the Nas server.
Even if browse the sub.
When I was using my kodi as server it can read, browse the sub.
Some body face such a problem and can help