you should give more info, as what program did you use to export meshes, which options you have activated, which fbx version you exported to, etc.
Maybe you export curves or modifiers in the mesh and substance does not handle very good? maybe I'm wrong but its a possibility.
Maybe your mesh have too small dimensions?
Maybe you are importing a highpoly mesh with 10million polygons?
are you sure your mesh has uvs assigned?
have you tried to rename files to eliminate spaces? I always use camelCase names.