Pretty sure they do know what you're after. It seems if you've been reading a recipe or said something in chat they start dropping. Having said that, when I've been looking for a single item it's never dropped for me, but today in an instance someone mentioned a dagger. It was the first drop after he said that. I've been farming mats, and I get several in the first half hour, then for ages nothing, then they start dropping. Blizzard definitely know what you're after and they do test your persistence. I'm sure of it. It seems like when you decide to quit one of what you're after drops. So you keep going. Try a rabbit's foot in your bag. Never know. Could work. But I'm not riding a frostsabre mount, I have no crimson or azure dragon whelp for a pet etc. Sigh