I dount think the shadows from the mountains ever hit the clouds.
The only shadows that hit the clouds are made by other clouds :-\
Is there something I'm missing?
To get the terrain to cast shadows on the clouds, you need "raytraced shadows" to be enabled in the cloud's node.
Thank you old_blaggard.
Be warned that it will dramatically increase render time when you enable this option and the majority of the time terrain is *not* casting shadows on clouds, or if it is the effect is subtle. Nonetheless there are certainly situations where it is useful to enable this setting.