When I create an item inside a chest using a command block, specifically:
/blockdata ~ ~4 ~3 {Items:[{id:diamond_axe}]}
the item appears in the chest.
However, when I hold that item and right click, it disappears. When I left click nothing happens, and when I attack mobs, it doesn't deal any extra damage. It is as if the item is not existing.
Environment
Windows 8
That's because it doesn't necessarily exist. The "Count" tag, if not defined, defaults to 0, meaning you have 0 of that item in your inventory. You need to apply the "Count" tag of 1 or higher: