Do you have all of your movies all lumped together in a single directory? Or do you have a separate folder for each movie and the associated subs for that movie in the folder? (/movies or /movies/Avengers - Age of Ultron (2015)?)
Do you run some sort of renamer against your media to get consistent names for everything, including subtitles? Are all of your subs named with the language code?
I ask these questions, because I run on Asustor hardware too. I have a separate folder for each movie title, renamed with FileBot for consistency. All of the subs or posters, or artwork, etc. for a given movie go into the folder for that movie and that movie alone. then, if you look at my library at the edit level, you would see it’s looking at a folder named “movies” and then each movie is it’s own folder under that. And all the pertinent files for a given movie are contained in the folder for that movie.
And that folder is named according to the name of the movie after FileBot has renamed it with Title (year)[resolution].ext so an English sub for that movie is going to be Title (year) [resolution].eng.srt. (So the folder’s name is Title (year) but no resolution, since I may have 2 or 3 versions of a movie in the folder to chose from.)
This works for every movie I have in my library that I have subs for. (Which is quite a lot, and I also have Spanish subs for most.) If I need to move a movie, I do it at a folder level and not a file level. Everything for that given movie is moved at one shot, including posters, artwork, subs, different resolutions, etc.
I actually go a step further with my folders, breaking movies into alpha based on first whole word of the title. So for the above example of Avengers - Age of Ultron, my structure is like this:
/movies <---- Library share point
…/A
…/Avengers - Age of Ultron (2015)
…Avengers - Age of Ultron (2015) [720].mp4
…Avengers - Age of Ultron (2015) [SD].mp4
…Avengers - Age of Ultron (2015).eng.srt
…Avengers - Age of Ultron (2015). spa.srt
This naming works for every movie in my library, with no problems what so ever. Subs are there and play perfectly every time on any movie in the library that has them.
I’m going to suggest that you give this a try for the movie “B” you are having issues with, first renaming it to what I outlined above, then create a folder matching the movie name, and putting it into the folder, keeping the name for the subs, as well. (Some downloads will have subs tied to that rip, and you can also include the resolution in the Sub name, if you need to. I find that this works just as well.)