If you're running a range team, there is currently a bug with IWT. Any class that does not have a range spec, (ie Mage, warlock, hunter) will only IWT until it is in range to attach the target ~20-30 yards. If you are trying to use it for looting, this bug does not apply. I had this problem and it was driving me nuts, though it is nice to not have my mages cluster on the unit they are attacking.