Shadowed Areas on Meshes not Changing in Brightness with Light Intensity and Ambient Intensity

I have a problem that is annoying me.
I have a terrain and meshes on the terrain, and a time system which changes the scenes lighting accordingly.

I'm using a directional light, and I'm using code that varies the directional light's intensity, and the scene's ambient lighting's intensity. My goal is to simply render the GameObjects in the scene darker when it's "night time" and brighter when it's "day time".

For the terrain, it's working as I intended it to. The terrain get's darker, including the trees and grass on it.
The meshes, however, don't work correctly. The shadowed areas stay at the same brightness/intensity no matter what the intensity is of the light or the ambient intensity.

I have attached some pictures to explain what I mean; in the Day picture, the shadowed area of the brick arch is the same brightness as in the night picture. Basically what I want to happen is for the mesh to become darker, just like how the terrain becomes darker.

I'm 'fairly' new at Unity, so I suspect the issue is coming from something super simple that I just have looked over. Any help would be greatly appreciated.

 

I did a work-around to get the effect I wanted and to solve the issue.
What I did was make the ambient source of light in the Scene Lighting tab set to "Color", and I simply change the value of the light, rather than change ambient light intensity from a skybox source.