Please clarify - by MAT file do you mean a shader node or do you mean a MAT pose file or do you mean a texture map? They are different things. If you mean MAT pose file won't apply, be sure you have a Figure AND a Body Part selected or the MAT won't apply - then you must save your figure to keep the material settings. If you mean a shader node, be sure your Image_Map node is connected to the proper input or it might be ignored when reloaded.