Question Unable to edit material for imported fbx

Starting out with Unity.
I've imported an asset pack with fbx and png textures.
Most of the models imported fine and the previews look good.
But some of the models just show the mesh with a standard gray material.
If I expand the good models, they have a nested Material and Mesh.
If I expand the bad model, it only has the nested Mesh.
So i created the missing materials, and am now trying to add the new materials to the Model; but it is uneditable.
First, i checked the file system to make sure i have read/write access to the file itself.
Second, i tried to create a nested material, but the Materials were only created at the Root folder level.
Third, i asked Muse, and it mentioned a submesh, which my models seem to have 1 submesh.
Fourth, Muse said i could check the import for a Read/Write checkbox.
If I enable that checkbox and Apply, i assumed i would be able to edit the material.
But no luck, I'm not able to select the Element 0 under Materials to switch from Default-Material.
Muse also mentioned there could be custom scripts/shaders; there are no scripts, not seeing anything obvious with shaders.
I'm running out of ideas, vs installing Blender, and trying to add the Material there before importing.
Thanks

 

Yep, that was it.
When I tried to edit another models scale, it let me edit it, but wouldn't let me apply the override with the message 'Applying to a Model Prefab is not possible"
As a newbie, when importing separate fbx & png files, i absolutely assumed that the Model composing those 2 assets would be editable.
But creating my own prefabs for the bad models were editable as you stated.
Not sure how to refer to imported model prefabs (not editable) vs prefabs i create (editable).
Will also have to figure out best practice of fixing assets before import vs creating my own prefab of all imported assets.
Thanks