Not every monster is going to drop something, it's always a random chance that something will be dropped....I've never gone into an area and had every single monster drop something, if they did, I'd be a lot richer than I am now
Loot scaling affects drops you get.
Just a suggestion, you shouldn't farm Trolls, they're not really worth it. There are MANY more places that are better, just check the Farming forums and you'll see.