Hi! Sorry for this, that's very common problem nowadays: damn Epic's ruined steamvr binding system in UE4.24 and now its behaving very random. I've added Index bindings but probably something gone wrong.
For now on Index it should work on Grip button.
If no:
Just go to SteamVR - Settings - Controllers - Custom - Edit this binding
and bind Trigger function
Ah! I managed to get it to work through the SteamVR bindings like you suggested. Great game, though it becomes too easy when you figure out you can create "trip wires" by throwing the TP sideways then throwing the other end the other direction. That said I managed to lock the game on one of the later waves with a large number of enemies when I ran out of TP and they clipped on the median.
Would you mind checking my game and see if there are binding issues for you? See the comment I posted. So sad that the only viable options for VR (Unreal and Unity) have such problems.
