Extremely Delayed Onset

Me and A couple of friends got together last night for a party and everyone was taking mdma. A couple of people were for the first time so it was an interesting experience. Anyways, my good friend and his SO took their pills at the same time as everyone else and got no real effects until about 6 hours later when everyone else had already left. Everyone else were feeling the effects within 45 minutes just about. They were good pills, "not from around here", neither of them are on SSRIs, and they both had 4 weeks between their last roll. Anyone have any clue as to what might have happened??