Try to run the game windowed. There currently is an unfortunate bug where vorpX can sometimes lose connection to the headset if a game gets minimized (e.g. via ALT+TAB). Will be fixed shortly in the next vorpX update.
If that does not help, it might me a hooking conflict with some program your PC that also hooks into games. Portal and Black Mesa ar both DX9, Arkham Knight is DX11, so maybe there is something that gets in the way with DX11. The trouble shooting guide on top of this sub-forum has more details on the matter.