Google Sketchup model problem in Unity

hey guys. im using the free version of google sketchup for buildings in unity. the free version doesnt have the right file format for unity, so im using a obj plugin. however, when it goes into unity, some of the sides are missing. do u think u could give me some advice about that? thanks

Import first to Blender and fix things there (flip normals), then just save the .blend file to your assets. Sketchup does not properly export normals.