I was running in to the same issue where…
Procedural Foliage calculates on a landscape if the layer limits were NOT set for the foliage types, but if I assigned the foliage a layer limit they would not generate.
After a lot of debugging I found that the Procedural Foliage calculations do not take into account any rotational transforms of the Landscape itself. This might also occur if the Landscape is scaled but I didn’t take the time to test this theory.
Bottom line, make sure you do not have any scaling or rotation on your Landscapes.
Hope this helps.