So you just have a SkyAtmosphere and no directional light or local area or point or spot lights in the scene? it looks like you might have a directional.
try it with a directional, SkyAtmosphere, skylight and volumetric clouds.
make sure the skylight is set to realtime and raytraced shadows enabled. make sure everything is movable in the lighting system and not stationary or static. make sure the directional light is set to be the sun and sky light. also, check the mesh distance fields and see if they are detailed enough.