Help with Black Powder ability

[Goatee-nethergarde-keep]

I’m currently levelling a subtlety rogue but I’m struggling with Black Powder when I do dungeons. Often I notice that I accumulate maximum combo points, hit Black Powder, but it’s doesn’t use any combo points, doesn’t land and sometimes I see Immune appear on screen, but not always

I’m sure I’m just being thick, but is there some mechanic I’m not noticing that prevents Black Powder being used at certain times? If I spam it, it often eventually seems to use my combo points and land. But not always

[Retier-ravenholdt]

don’t have enough energy maybe? black powder just needs points and energy. no range, target or positional requirement (to cast it anyway, to actually land on targets it’s different).