Some games, like League of Legends or Apex Legends, will run in elevated mode. You will see the UAC prompt at time of install or first launch of games that do this. It looks like this

If your game is not recording, and not being noticed by Plays, it may be running elevated. In order to record games in this mode, you will need to enable the "Record elevated games" setting located in Settings > General tab of Plays. This will cause Plays to ask to be restarted. Once restarted, Plays will show you it's own UAC prompt which you will need to say "Yes" to in order for recording to work. Don't worry, this mode will also capture games in non-elevated mode.