Check Subtitle Settings and Configuration. It may seem obvious, but your Prime Video app’s settings menu is the first place you should go to try and fix a subtitle issue. Turning the subtitles off and then on may recalibrate the subtitle display and get it to display properly.
Check if subtitles are working with other TV shows or movies. Since the original content creator typically encodes subtitles, there’s a chance the Prime Video title you’re watching doesn’t support them. Try turning subtitles on for a few different TV shows or movies to ensure this isn't an isolated issue. If they still aren’t working, it’s likely an app-wide issue.
Restart the Amazon Prime Video App. If subtitles aren’t working properly after making adjustments in the Settings and Configuration menu, your next best option is to restart the Prime Video app. This will clear up some of the app’s temporary cache, which could fix subtitle issues.
Depending on the device you’re using, exiting out of the app may not actually close it. If you can’t force the app to restart, you should restart the device instead.