I have an extra Glenmoril witch head in my inventory, but nothing happens when I activate the shrine. I tried to do this while on the Purity mission for Farkas. Tried to activate the shrine before talking to Farkas (after fighting his spirit wolf) and after the quest completed, but nothing happens at the shrine. Did I miss out on a chance to cure myself?

Did you also "cure" Vilkas?

That was the ticket... I did get the option after curing both brothers. Thanks.