This is likely the software bug ToyKeeper mentioned in another thread.

Basically Anduril checks for the battery level and other stuff every x seconds and if you try to click your light on in that exact moment it won't react. Happens to me too every couple days/weeks depending on how often I use it.

The fix for this implies a larger rewrite of the Anduril code and will probably take a while. I'm not very happy with this bug either.