Hello,
I suspect that the preview mesh is null. I don’t believe you can view a skeleton without a preview mesh selected. What happens when you try to select your mesh from the preview mesh dropdown?
Also, when you copy move your assets, are you doing so from within UE4? Manually moving assets outside UE4 is not a supported feature. Even if the uasset is placed in a new folder, it will not show up in the editor if you did not move it from within the editor.