View your model in Monochrome mode.
If you can see blue-gray faces, then those are the backs of faces.
If you have applied materials to those, then they will not appear in the OBJ version, as only front-face materials get included.
You can Reverse wrongly oriented faces by selecting them and using the context-menu tool.
You’ll need to reapply the materials.
I wrote a tool to automate this - look here - SketchUp Plugins | PluginStore | SketchUcation
If finds faces in the selection which have materials on their backs, then reverses the face and reapplies the material that was on the back onto the front, maintaining any texture UV mapping to suit…
Well modeled geometry should only show the off-white front-faces [the actual color depends on your Style settings]