Frank, here's a file showing the problem (not the original but that don't matter).
Interestingly, I've noticed that the populator only fails to populate when the min slope box is checked in the distribution shader , reporting 0 of xxx instances. This is at any min value. Inverting the density shader seems to work, but what is it the inverse of?
I made this scene very quickly by first creating a TGD with a working population that uses a distribution shader as it's density shader.
I then saved the object pop as a clip file, started a new project and inserted the clip file from the first scene.
Not really a problem for me, just thought it should be reported as a bug, but no response from PS.
Thanks for your interest.