This has apparently been going on for years, but when you do the quest, after a while the items collected do not count. (and, yes, I know each group only counts for one) You use the Item, collect 1, and go on to the next group. After a while, it quits counting towards the quest objective. (I got up to 14, abandoned. Tried again and got to 18 before it bugged out.)
the top couple of comments on wowhead should help with a work-around until it gets fixed