Hey.
I am hitting my head against the wall, because such problems really frustrate me. I am like 3h yet trying to fix this.
Some objects on the scene has like black color, that is, the material is not visible (only on Mobile).
Problem description:
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I tried many different materials, or no material, but it has no effect.
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When i am trying to debug it on PC (because the “building & run” is way faster than on tablet) with Mobile preview, i see red message on top The lights needs to be rebuild, but how to rebuild them? I am hitting Build button in editor…
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When i run Mobile preview then for small portion of time i see the material on the object, and then it like fades out. Like if the material is correct, but it is out of range of light area. (like dark side of moon). But i tried to move light source (direct lightning) or move the cube (its only cube) and it is still black. Moreover, some objects around are ok, which breaks this theory.
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On PC version (PIE or Stand alone version) there are no such troubles
Settings which can theoretically affect the problem:
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I am using extra UV’s on the mesh as light map index, that means #1, but this settings should not be problem.
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The mesh is scaled up by 75.0 on all axes (due blender difference in units)
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I am running UE from C++ (that means it is C++ project)
Same problem but bit different settings:
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I have some Actor created in C++, constructor is empty, its basicly just inherited actor, with one my custom method
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If i create blueprint which inherits this actor, and place it on the scene manually in editor, material is rendered, if i spawn it via blueprint with SpawnActorFromClass() it is black (only on mobile; object is again scaled up like 40x, trie many materials - the one i need to use is instance of dynamic material, but i tried also another…)
What could be the problem?
- Some mistake in C++ (i omitted something? - but the first case object is not C++ Actor, but just static mesh)
- Some settings of Direct lightning?
- Packaging settings?
Dont know… 
Thx
L.