Hi,
I just checked one of those models on fbx base within Blender.

The issue is, that any single face is treated as an object. This is something really mad. Which litteraly means: one Board (shelf) has 6 faces... It's a way easier to build something like this from scratch.

Hi Baldy,

There are two basic questions on Your Model:
1. What render engine do You use in Blender (mght be also fine to know what Blender Version)
2. You used the extrude for the rail, as a modifier?

Both things will make it uneasy to get the right OBJ (MTLs) out of Blender.

So first rule of ...