I tested building generation with your settings and it works correctly on my side. Screenshot attached.
So the problem is elsewhere.
Please attach a full screenshot of RWT window (including area) so I can test it with all settings identical to yours.

I have been testing this asset for a few days now and it works great for what I want to do with it.
Now I want to create a flat terrain and that buildings are getting generated, but when I create a flat terrain, the buildings won't generate at all.

I will add an image of the settings when generating the terrain, and maybe some of you might know why buildings won't generate.