Provided you added your subtitles correctly there's a couple of things to check if you cannot see subtitles during playback.

  1.  Make sure you used and .srt subtitle file if you added them manually
  2. If there were already embedded subtitles in the file but they didn't appear in the converted file. It's very likely the embedded subtitle weren't text based but instead image based ones. These are ignored and won't appear in the converted file. Try added a .srt subtitle file instead.
  3. Many video players don't display subtitles by default. You need to select the subtitle track in order for them to appear.