Hi!
if I may make a few suggestions that include wild guesses?
I am guessing that you have used Luc's file as a basis. If that's true, the problem with that is
a. no meta cloud... hence you'l have multiple clouds in the backgrond, but the settings are not tuned to make them look good (not possible just with Luc's file). If you look through Luc's cumulonimbus images... he always took precautions that the viewer cannot see far away parts of the sky. that' the trade-off-
b. Luc's settings are good for a relative close up, and no so good for distant shots (without specific tuning)
c. that the clouds have that block-like bottom, that is not looking realistic. You need a mix of a cauliflower top and a more whispy and soft bottom, and an overall shape that doesn't look clumsy.
If I remember correctly you have my cumulus pack. I will soon update that (and provide it free to customers) with a specifically tuned scene for distant cumulus shots. There's already one for distant shots in there, but I am guessing that's not the look you were shooting for, but rather a look like the last image I posted here.
Forgive me if I'm wrong, please.
cheers,
Frank