It isn't bad luck... Green items have a very low drop chance in a party, the more people the lower the chance. Also, I think henchmen take up more loot then people.

If your soloing, you have a lot higher chance. I use the E/A Shadow Form/Sliver Armor boss solo, which can be found in the Assassin forums on GWOnline.net It is in the Assassin forums, since it originally was A/E. The topic name is something along the lines of, "Tired of all that 'Assassins can't solo garbage'?"