Each boss drops 9 items, so if your looking for a specific item, you really cant even begin to expect it will be yours until #9 if you havent seen it yet. Running 18 times and not seeing that drop is no different than flipping a coin and having it land on the the same side twice...its not that uncommon, even 3-4 times even tho the odds are 50/50. Running for a specific item is what alot of us like to do, but realistically it's not a smart move unless your willing to do 50+ runs ahead of time, considering there might be another of ur class that wants/rolls on said item.