I'm guessing your hotspot is set somewhere (like at the feet of a sidescrolling platformer character)
I had the same problem, but then realized the "invisible particles" for LOS detection are likely spawned from the hot spot and were therefore immediately getting destroyed (becasue the hotspot is touching a solid object.. IE, as soon as the invisible detection particle gets spawned, it gets destroyed!)
I moved the hotspot up to where the eyes would be inthe sprite and it now works fine.
is this the case for you as well?
I wish we could set the origin point or "eyes" to another point.