Hello Priareos,
I was able to get your foliage shadows functioning on your map to a good degree. If you want darker shadows, change the light color of your Skylight to black. Skylight is used for Ambient Lighting, so when it is white, it tends to wash out the shadows and your scene in general. Also by increasing the intensity of your Directional light a bit as well you can get a nice contrast between your meshes and your shadows.
I increased the size of your foliage rocks to show you how the shadows have changed. I also noticed you have multiple types of foliage settings for the same mesh, which can cause confusion later down the road if you are trying to edit the instance settings of those particular foliage.
Here are some screenshots of what I was able to get by changing the Skylight color, increasing the Directional light intensity, and increasing the size of your foliage a bit.
Skylight Color Change
Directional Light Intensity
I suggest making sure you have all of your rocks on one foliage setting with a good min and max size range and density so you get the appropriate scattering and the correct random sizes as you desire.
Thanks again,