If the StreamVox teleprompter works over your desktop, browser and Discord, but not over a game (League of Legends, Valorant, etc.) — this is not a StreamVox bug. It's a Windows limitation.
Discord, OBS and GeForce overlays hit the exact same limit.
Why this happens
When a game runs in exclusive Full Screen mode, Windows gives that game the whole screen directly. No normal "always-on-top" overlay can appear over it.
Fix: set the game to Borderless mode
Run the game in Borderless (also called "Borderless Windowed") instead of "Full Screen". The game still looks fullscreen, with no real performance loss — and overlays work again.
League of Legends
In-game press Esc → Options → Video, set Window Mode → Borderless, click Apply.
Other games
Look in the game's Video or Display settings for Window Mode and choose Borderless / Borderless Windowed (avoid "Fullscreen" / "Exclusive Fullscreen").
After that, the StreamVox teleprompter appears on top. If it's still not visible, make sure the teleprompter is enabled and dragged onto a visible part of the screen.