Is it happening to all shows and movies with the subtitles "S" symbol next to them or only certain TV shows or episodes ?
If it's the former can you confirm the nowtv playback device, where i know the nowtv boxes and stick have in their menu settings a captions option which needs to be enabled for subtitles to show on screen.
If it's only certain TV shows or episodes where the subtitles are not showing where they are supported by the subtitles symbol, then @Mary-E of the NowTV Content Team can then look into this further for you.
I don't own either a Samsung or LG smart TV with the inbuilt App, so i won't be much help sorry.
A couple of Samsung and LG smart TV owners have mentioned that they can not permanently enable subtitles on the Nowtv App recently.
Where they have said that they can temporary enable subtitles by pausing the nowtv content and enabling the bubble speech icon on screen, but every time you play a different episode or movie you need to go through the same repeat process again which is a bit of a pain if you ask me.
I believe the NowTV Team are looking into it, but in the meantime i would double check if there is any captions or subtitles settings inside your television menu settings which needs enabling.
Suppose you could try live chat to see if they have a solution or if they can give you a time frame when this will be fixed.