Material doesnt appear in game after packaging

Hi guys!
I created an interactive archviz project, with options for changing mesh materials. When I run the simulation, everything works fine. However, when I package the project and run the .exe, a single material simply does not load correctly (the stair wall). All materials can be changed without any problems, but this specific material, when selected using the menu, is all white. Does anyone know what it could be?
thanx in advance!!