Issues with editing material list when editing and reimporting player model

I have been using a personal basic animated character model for prototyping. After finishing some scripting I modified said model, needing multiple materials applied to different parts of the mesh (metallic armor, rougher clothy parts etc…). The issue is whenever I add a new material in blender and assign it to certain faces, the specified faces straight up do not render inside the game engine. What am I doing wrong? Thanks!