Apparently there's a bug that causes Fossilize to become unusable. I spent some time dueling on my magicka DK and came across this bug:
https://youtu.be/G6xAE7UyzZo
The bug doesn't resolve itself until you relog and has happened to me in both duels & BGs so far.
I'm playing a destro/resto magicka DK (in case it has something to do with animations or something).