Below is a checklist of steps to troubleshoot the issue:

1. Check if subtitles are working on other devices.
2. Check if the streaming service has subtitle support enabled.
3. Check if the TV subtitles option for the streaming service is enabled.
4. Make sure the TV firmware is up to date.
5. Ensure the connection speed is sufficient for streaming.
6. Uninstall and reinstall the streaming app on the TV.
7. Check if using a different audio output solves the issue.

If the issue persists after following these steps, you may need to contact your Smart TV manufacturer.