Imports are not working.

Hello,
I ran into an issue with my imports, I'm currently creating one of my projects which is dependend on another one.

In the project I'm working on, I'm trying to import classes and functions from others but they're not being found by the system, even though I can compile the whole project and it works perfectly fine. The classes are highlighted as red and as “Unresolved reference”… How can I fix it? 

I have to add that everything used to work when I was working on this project a few months ago, the issue started just in December and sometimes it fixes itself. It mainly comes back whenever I add the compiled file to the project again