Not sure either. You should always name the exact Challenge. I can´t imagine he talks about the Wandering Servoskull. But I have in some occasions missed out on it.

Deliver in X number of Fragments for Custodian Rebus perhaps? That one can sometimes be a bit tricky if you are just a bit short, and when you need to wait all the way to next Weekend.

Is it the 75 mins to complete void crusade one? 

If so use a guide for the void crusade you are trying to do. You can google Inquisitor Martyr void crusade guide, and find images showing the layout, and cogitator/secret mission locations. It also helps you avoid insane difficulty increases.