Blender FBX to Unity - Only importing parts of my model

Hi, Thank you for taking the time to look at this.

I have Youtube Searched, Googled, Smoke several packs of cigarettes, and slammed my head against the desk a few times.

I made two different models in Blender. Both are steps. 1 (which is geometrically simple and square, and one that is the same model, but broken up along the edges a little bit.

photo #1 - In unity The simple steps in the background have imported correctly. The more detailed steps in the foreground, only imports as a partial model with invisible parts.

I have exported both of these objects the same way. They do have different UV Texture Maps, that I hand traced the seams. (Not Smart project). The object is rendering fine in Blender, as you can see. I've even tried remaking the model. I do not believe there to be any n-gons in this model.

Any ideas why this is happening to me and this model? I'm very new to this type of trade/hobby. I am learning still, and getting over this hurdle would certainly be awesome to me. If someone could please hold my hand through this problem with a decent explanation I would truly be most appreciative.

Thank you.