I was trying to get the "Explosive" challenge but when I had five creepers stacked it said I was missing two of them (see below). I have no clue as to how it could be fixed.
A stack of Creepers counts as one Creeper. You need to have an individual 3 near each other *before* they stack. I believe they begin to stack when 5 are near each other, so you have some leeway. Good luck! cheers~
