I own 5 items of the Defiler's set. I am wearing 4 of the set: 2 rings, 1 necklace and boots.
If I equip my Defiler's Bow, the set stays at (4/5), it doesn't change to (5/5). The Defiler's Bow has the correct description i.e. it lists all the set bonuses, so I know it's part of the Defiler's set.
I also have a Bow of Endurance which was equipped when I got the Defiler's Bow. If I unequip the Bow of Endurance, it says I have (0/5) of the Endurance set equipped. But, if I equip it, it goes from (0/5) equipped to (2/5) equipped for the Endurance set (and I don't own any other items in the Endurance set). I can sit there selecting and unselecting the Bow of Endurance, and the screen toggles between (0/5) equipped and (2/5) equipped.
So, it seems that my Bow of Endurance is worth 2 of a set and my Defiler's Bow is worth 0 of a set. It seems like both my bows got bugged so that one of them has the bonus flag for both of them.
I need my bows fixed so that they are both worth 1 of a set, so I can wear (5/5) of the Defiler's set.
Thanks.