1. Here's how

There is an upper limit to the number of Foliage leaves per polygon for a model. On a large polygon without any sub-divisions you won't see as high a leaf distribution.

1.1: If the model contains large polygons, simply sub-divide the polygons that lack Foliage density and re-import the model in Lumion:

Before sub-dividing the large polygon, the Foliage leaves are far apart:

After sub-dividing the large polygon in SketchUp, the Foliage leaves are distributed more densely:

Note: adding lots of Foliage can have an impact on screen redraws (FPS) and render times. Please use sparingly.