If you are unsuccessful many times in a row, it is time to wait for them to de-spawn. Both despawn around 8 am game time, then respawn with a chance to have the item you need.
I was successful on both last week after two or three spawns. Helps if you do not have some one else looking for the same thing. Curiuosly enough, I had to compete with some rogue who felt the need to shroud to a goblin rogue since the shrouds get automatic 200 in pick pockets. Have some pride, sheesh.