Why are parts of the image rendering so grainy or pixelated? (Blender 2.79)

I am trying to render something I’ve been working on, but every time I try, it doesn’t fully render. I’m using cycles render in version 2.79. In the center of the piece, there’s a cloud. The cloud barely renders at all. There are other areas around the image that didn’t render properly, either. The main problem area is the cloud. I used the cloud generator add-on.

When I say "doesn’t fully render", I mean that some spots all over the image are super grainy and pixelated. Does anyone know how to fix this problem?