On various quests, the drop rate on quest items needed for completion (Wasteland Lion Paws, Ksavari Hides, etc.) seem to drop dramatically when in a group. when a quest starts multiple members of a group can receive the needed items per kill on a fairly regular basis, but after a while the rate drops to none of the members getting any items for an hour or so. I've seen this with a few quests, on various characters.