You didn't say which type of atmosphere you are using in Vue 6. You'll get the best results with Vue 6's new "Spectral Atmosphere".
 
I suggest you start a new scene with that atmosphere type and experiment until you understand it - then convert your old scenes to the new Spectral Atmosphere type - with it's beautiful clouds.

At the same time, you might want to practice with the Vue 6 metaclouds.

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