There may be another problem than how to make clouds.
First, you’re using artboards. They have a tendency to absorb way too much medium white. That makes layering of paints difficult and translates into all kinds of problems.
You might try using a lot less medium white on your artboards. If that doesn’t work, than try the next suggestion.
Now…. artboards require different levels of pressures than using on canvas. You may be pressing way too hard. Try extremely light touches with your brushes.
Consider the following as a low cost alternative to using canvas.
Buy a pad of canvas papers from AC Moore. The 16×20 or 18×24 whatever you like. And then buy a canvas of equal size. Thumbtack a sheet of canvas paper to your canvas. Paint your painting and then thumbtack a new canvas sheet to your canvas each time you want to paint one.
This gives you the feel of canvas at a much lower price. Then I just thumbtack finished paintings to a basement wall until they’re dry.