Foliage - can't paint on wall?

I can paint the foliage on the ground floor.
But when I try to paint on a wall - nothing happens.

Is there anywhere in the settings a limitation?
Degree? Angle?

What am I missing here?

Thx for any little help!

figured it out. if someone else is searching:
Ground Sloap Angle: Increase to 90 degrees

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