This feature often trips me up, so I thought I'd post it here...
When I duplicate an artboard an artboard and all its contents, the duplicated artboards contents are not visable... until you select the Artboard tool, click the cog - "set artboard behaviours" in the top bar, then tick the "auto size canvas" option.
This behaviour is confusing to me. when i'd expect to see that new artboard just duplicated.
Here's a thread from another forum with the same info: