Damageing item

Hello, I am trying to make a custom item that can pick up a type of block and store it.

Currently, when I use it, the log displays 0 15 when it try's to pick it up.

The damage bar appears for a tick then disappears. I assume this means the damage is being reset.

I don't know if this matters, but the item has nbt

Code: (In onItemRightClick)

            		int damage = itemStack.getItemDamage();
            		itemStack.setItemDamage(damage + 16-world.getBlockMetadata(x, y, z));
            		log.info(damage+" "+itemStack.getItemDamage());