How to force Plex to load missing .srt subtitles for all library

Hello,
I am Italian and I see every shows in English with subtitles.
From time to time, Plex loads some episodes without subs, even if the file exists and it is well formed in name and content. I am constrained to use alternative player (Infuse) in those cases. Sometimes I solved clicking on "update" on the server page of the single episode, but it is awkward and doesn't work every times. I also tried "update all" from series root directory, but nothing happen (the process takes about 30 minutes). Why Plex ignore some .srt and how can I force it to read them for all episodes in my library at once in a single operation?