I just learned another shout that I want to unlock, and I have killed enough dragons to have five extra dragon souls, but I can't unlock it in the magic menu ("p" by default).
I have unlocked some other shouts previously, but the two shouts I haven't unlocked yet won't unlock when I click on them.

This is the first in-game problem I've encountered (yet), but it's so very frustrating!

I've tried closing the game and loading, and I've tried loading a previous save, but neither works.. Does anyone have a fix for this in some way?