so im at the harbingers flame and d have the witches head but for some reason it wont let me activate the flame to drop it into the flame, is this a bug or am i not doing something right?

It may be a bug, from what I remember you walked up to the flame and text appeared giving you the option to activate the flame when you were close to it. Activating the flame asked you if you wanted to cast the witches head into the flame. Other than having the head in your inventory and pressing A when given the option while your next to the flame there isn't anything you have to do to prompt it. I don't really have a fix other than loading an earlier save, perhaps someone else could be of more help though.

Try dropping the heads, it worked for me once.