This seems as good a thread as any to ask this.
Doing Terror from Beyond story mode last night, we twice wiped due to the same thing: the boss started channeling Scream at the other tank, I taunted during the channel, the boss ignored me and killed the other tank when he finished the channel.
My taunt did go off, definitely, it just ignored it. Bug or is there something we're not aware of? As far as I know and as all guides say, I have to taunt during Scream and when the boss finishes Scream, he'll turn to me so I receive his next attack, not the debuffed tank. Is that correct?
EDIT: our other tank also swears he did not re-taunt the boss.
Did the first tank immediately die after scream finished casting?
If so, this is probably due to the damage scream does when it lands on the tank. If the tank getting screamed is not topped off, he could still die from this, which may seem like your taunt didn't work.
So to be clear, when scream is finished casting, it will land a big chunk of damage on the first tank. It then switches to the second tank that taunted and continue regular attacks. Healers should be prepared for this.
On HM, this scream usually does close to half my tanks HP in damage (aprox 11-13k). It's probably weaker in story, so your healers got to step it up. Or maybe your tank isn't moving out of the spit and taking unnecessary damage.