Certain objects are black no matter what.

Very odd things happening in Unity. I built a scene using skyshop for the fancy lighting. It looked beautiful. Published it to phone and realised it was too intensive. Probably due to me using a lot of transparent glass in the scene maybe. Anyway, as I start to optimise and mess with lightmapping, using more standard shaders some objects go solid black in the build. As if the material is corrupt or something.

Now I've removed all skyshop elements and I'm trying to run the scene using just standard shaders.

But without warning some of the objects go black, even if there is another object using the exact same material working fine in the same scene. I even have a button which has a multi material on it, both materials use the same shader (default bumped diffuse). The "off" material for this object (it's a button, a 3d asset, not an interface element) works fine but when the code switches it to "on" and changes to the other material (which is using the same shader) then the new material is solid black. The ONLY thing that is different is the texture jpg.

Yesterday I spent a LOT of time changing the lighting, turning some/all of them off and on. Sometimes the materials would show, sometimes other objects would go black.

There is no lightmapping in the scene, but I did use lightmapping earlier in the project and then removed it so I wonder if there's some hangover info confusing things??

This is the final hurdle to me publishing this app and any help would be appreciated.