Mmm...not sure. Do you have the default foreground and background colours set (black and white) before you apply the clouds? Are you applying the clouds to a new layer? Have you played around with the "Blend if:" settings? There are a number of aspects to look at.

Hey Tamlin...

Looking at a fog tutorial you posted over a year ago (Photoshop Tutorials - Realistic Fog & Mist) and i really love the effect you achieved. however, i'm not able to go past step 4. once i apply the clouds filter, i have a solid grey mass, and further steps dont seem to disperse the clouds like it has appeared to work for you. Any idea what dumb mistake I'm making? Thanks so much for your help!

-Night