scatter on surface doesn't work

[enzo_87]

Hi everyone,
I'm in the middle of creating some vertical walls with the vertical assets from the library.
I have several walls that are facing in different direction and because of this I'm trying to use the UV set up.
Problem is, FP icon is becoming yellow but I don't see any object scattered.

I thought I was doing something wrong, so I tried with other presets(grass) on a plane surface, with either xy grid or uv, but it's not working.
I think I'm following the right configuration for using and at hits point I'm not sure if I'm doing something wrong or if there is a problem.

Is there a known issue?
Something I should try or attached so as you to check?

Thanks in advance,
Lorenzo