Adding foliage to curved landscape?

I am unable to add any foliage in this area using foliage editor, but I can however add it manually. That takes more time ofcourse
Is there any way to add it with foliage editor? And why is this a problem?

Look into your foliage’s “Ground Slope Angle”. The default is 45 degrees and if the slope of the landscape is higher than your max angle, you wont be able to paint on it.

Picture of the setting:

That’s exactly what I was looking for. Thank you good sir !