Even though I browse their fandom pages on the site (meaning they clearly exist), I can't add the fandoms for Rune
Factory 4 or Stardew Valley to my fandoms. They don't even show up on the app when I go to Add Fandom.
Also, are the fandom pages for TV shows in a different place? I like using the House, M.D. and Law and Order:SVU pages and would like to add those, too.
A vast majority of wikis actually don't appear on the FANDOM app and they won't really say what their criteria is to get it on there.
Basically, don't be surprised if a topic doesn't appear on the app, because most don't. From my experience TV shows and well-known movies tend to get favoritism over even very popular book series or games.