I have a raspberry pi 3 with OSMC for operating system. I have wifi on the pi but 2 bars of strength. the subtitles give message " subtitles not found. Maybe need a stronger wifi signal?
As I don't know if we are talking about subtitles already downloaded or not:
Maybe check if there is a subtitles file to start with?
Maybe check if your subtitles services are configured correctly?
Maybe check if there isn't a problem with the subtitleprovider?