I usually have no problems with itSeez3D scans ( OBJ format ).
Recently, however, I noticed that Unity has trouble with them. Some faces are missing, or maybe they have incorrect normals:
You can fix this in Meshlab. Use Filters → Normals, Curvature and Orientation → Compute Face Normals and re-export the OBJ.
I don't know Unity very well, so if it's possible to do this natively, let me know!