Black grass close to other meshes

Hi friends, I’m building lightin for a landscape with grass tool material (not foliage tool). I see some areas close to other meshes where the grass is black…
I’m using static illumination and static meshes because I need this kind of lights.

I’m using a light sourceand a skylight with a image.

I saw some post with the same problem in the forum and outside this one, but nobody solves it.

You can see black grass close to rocks for example in the follow image: