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I usually send a packet explicitly to force the update to the client. This is because the order you do things to create the packet then add the NBT might miss the automatic update packets. For example, I think if you create an Item and add the NBT and then put it in a slot you will probably be automatically updated, but if you take an item already in a slot and add the NBT then probably not. You can use a vanilla packet, but the packet you use depends on where the item is currently residing. For example, if it is in a GUI then I think you would send an SPacketWindowItems packet. If it is is some of the player's equipment (worn armor or something in either hand) then you would send an SPacketEntityEquipment packet and if it is in the hotbar then use SPacketHeldItemChanged. Now, I think it is supposed to automatically update in most/many cases but personally I've found sending an explicit packet solves these sorts of things sometimes. Other than that, I would say just trace the execution including any packets sent, looking at the payload involved to see if the information you're expecting is there.