Getting undefined reference when compiling for Windows, but not for Linux

I'm facing an issue: I have two codes, one is a lib and the other is the main project which calls for that lib. When I compile the code in Linux Ubuntu, everything runs fine. But when I compile the same code on Windows (using MinGW), I get an "undefined error" for one of the functions I'm calling.

I already checked the add library paths and everything is OK. I also checked content on Internet such this SO question, but nothing helped. Any guesses why this behaviour would be happening?

Btw an old version of this project and lib was compilable on Windows before AFAIR, but I lost the backups due to problems maintaining the SVN.