I've got a simple, 1 model population and I'm trying to restrict it so that it only appears above the water (sounds reasonable).
I plugged a distribution layer into it that only sits above the water, and hit a quick render, and the entire world is still covered in trees.
This happened to me recently when I was messing with more advanced object placement, and I thought then that there must be a problem with the node layout, but now I'm beginning to think that there might be a more fundamental problem with my method.
The weirdest thing is I've gotten them to work before, but they don't know.
I've attached the tgd for you.