I had this and made myself look a right twit in ooc figuring it out

Voice of Clairvoyance includes the haste component, so when you buff one, it overwrites the other. That's why if you buff from Sune they seem to stack, because she gives you Voice of Quellious which has no haste component.
* edit * so.. just noticed your posting date. whoops. Maybe someone else will have the same problem and this'll help them though..