problem when exporting from Blender 2.93.1 to Unity 2019.4.29f1

hello, the problem is that within blender the model looks perfectly but when exporting it and putting it inside Unity some parts are seen as transparent. especially the shoulders, and the part of the armor of the legs and boots

the materials don't bother me so much I'm just going to put them back in unity

I attach photos of how they look in Blender and then in Unity