Please use the DVDFab preview window to playback the movie, disable Subtitle and see if you can get those "forced subtitles" shown up or not. If you don't get them while Subtitle OFF, then they are not Forced subtitles and once you tick "Forced only" you won't get any subtitle for playback.
You can playback other subtitle stream and see which one contains the subtitle you needed, and then pick that one for the conversion.