I performed the steps mentioned in the pinned post (v 184.108.40.206
Yesterday when I was questing, the buff monitor stopped working for my priest, but only for Power Word: Fortitude.
The bar still turns white, indicating a missing buff, but left click won't do anything.
This only happens for this class, for this specific buff.
Removing and adding the buff to the list didn't help either.
I had the same issue. I found out that you need to have the highest version of the spell learned. I still had Rank 1 for Power Word; Fortitude, but I think Healbot was looking for Rank 2. As soon as I learned that spell, I was able to use the Buff Monitor and apply the buff again.