Stop wasting your time with them. The drop rate for any Exotic in ROC is ridiculously low. It's easier to do Patrols, as you can earn reputation faster and you get blue drops about just as often. This brings up your rank with the Cryptarch and whatever Faction you're working with, and you get the packages. These commonly contain Legendary engrams, which can then turn into an Exotic rarely. It's my method, and I've gotten a few this way. Personally, I got my Icebreaker from Atheon on Hard Mode.