Hi there!
try this:
https://docs.unrealengine.com/latest/INT/Engine/Landscape/QuickStart/4/
Sometimes at my Terrain i have to delete the ground that i dont want. It seems, that you paint over the other layer. But this don’t delete the other one. So you have to make sure, that you only paint one layer. Over the brush Setting you can choose the hardeness of the brush.
Hope this helps.