Child-theme not woking in theme/includes folder

Hi, been using the child-theme structure and it's been working nicely, as in theme/header.php and theme-child/header.php - this works like a charm so I can do mods on the theme-child/header.php without having to touch the main (out of the box) theme/header.php.

I did the same scenario with theme folder structures that use subfolders: For theme/groups/single/group-header.php and theme-child/groups/single/group-header.php, this too works like a charm. However, when I tried this on the file main-navigation.php, I'm hitting a slight bump on the road. When I added the main-navigation.php file to my child folder and modified the theme-child/includes/main-navigation.php - the changes did not propagate and do not show up on the browser. It looks like it's still looking at the main theme : theme/includes/main-navigation.php vs the theme-child/includes/main-navigation.php. When I take the file out (as in remove the file in the main theme/includes folder, the php engine complains, but when I do this in the other scenarios I described above, it does not complain and uses the child theme php file even if there is no parent theme php file). Any ideas? Thanks.