Most subs are available in one or more parts (=files), depending on the movie being in one or more parts. So is your movie also split up in two files=parts?
Do the names of the movie-parts match with those of the subs-parts?
If movie is in one part and subs in two, you should select another sub (in one file).
(Alternatively:
How can i join two subtitle files?)
PS:
If BS.Player cannot find appropriate subtitle-parts for a movie, you could try to find them "by hand". See item (9) of my Help2Help-post.