Before entering Sovengarde to kill Alduin, I did some research regarding Paarthurnax and whether or not killing him was a good idea. I decided to let him live so the Greybeards could assist me with finding two more shouts - it's all I need for the achievement.

I returned to High Hrothgar after finishing the main quest, and the Greybeards aren't there. I've looked it up on Google, but I haven't found an answer.

Is this a bug? Is there anything I can do to trigger the Greybeards returning?

Thanks in advance.

PS - Could this be a bug similar to Dremora and ore veins not respawning on my current game save?