I used to think the same, but apparently ICDs are a thing after all. Something with Shadow Oil thatupsets retribution paladins.
It could actually be not ICD but that weird issue they blueposted about recently - that some items won’t proc if you’re on a GCD from using a spell (Coldrage Dagger or Frost Oil won’t proc if you had just cast Abolish Poison - these sorts of Viscidus things, also might be relevant to Nightfall not proccing a lot).
To OP, warlock discord may be a better place to ask.